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PROCEEDINGS

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CALIFORNIA ACADEMY OF SCIENCES.

Third Series.
Math.-Phys. Vol. I, No. 7.

The Apparent Projection of Stars upon the

bright Limb of the Moon at Occulta-

tion, and Similar Phenomena at

Total Solar Eclipses, Transits

of Venus and Mercury,

Etc., Etc.

BY

George Davidson, Ph.D., Sc.D.

Issued December 1, 1900.

SAN FRANCISCO:

Published by the Academy.

1900.

PUBLICATION COMMITTEE.

Charles H. Gilbert, Chairman.
William E. Ritter, G. P. Rixford.

\

EDITORS OF MATHEMATICAL-PHYSICAL PUBLICATIONS.

Irving Stringham, E. P. Lewis.

PROCEEDINGS

OF THE

CALIFORNIA ACADEMY OF SCIENCES.

Third Series.

Math.-Phys. Vol. I, No. 7.

The Apparent Projection of Stars upon the
bright Limb of the Moon at Occupa-
tion, and Similar Phenomena at
Total Solar Eclipses, Transits
of Venus and Mercury,
Etc., Etc.

BY

George .Davidson, Ph.D., Sc.D.

Issued December 1, /goo.

SAN FRANCISCO:

Published by the Academy.

1900.

Digitized by the Internet Archive

in 2008 with funding from

Microsoft Corporation

http://www.archive.org/details/apparentprojectiOOdavirich

|00 o 3^

THE APPARENT PROJECTION OF STARS UPON
THE BRIGHT LIMB OF THE MOON AT
OCCULT ATION, AND SIMILAR PHENOM-
ENA AT TOTAL SOLAR ECLIPSES,
TRANSITS OF VENUS AND
MERCURY, Etc., Etc.

BY GEORGE DAVIDSON, PH.D, SC.D.

Read before the California Academy of Sciences, August 9, 1897.

CONTENTS.

Page.

Introduction 64

Geodetic Observations in the Sierra Nevada 66

Visibility of Jupiter's Satellites 66

Conditions of Steadiness and Unsteadiness of the Atmosphere 67
Observed Occultations of Red Stars at the Bright Limb of

the Moon 73

Other Observed Occultations of Stars by the Moon, and of

Stars and Satellites by Jupiter 75

Similar Phenomena at Total Solar Eclipses 81

Similar Phenomena at Transits of Venus and Mercury Across

the Sun's Disc 82

Instructions for Observing the Transit of Venus 85

Curious Descriptions of the Phenomena 87

Errors of Observation 88

Observations of Solar Eclipses under Different Atmospheric

Conditions 89

Graphic Exhibition of Unsteadiness 92

Official Recognition of the Factitious Borders of the Moon

and Sun 93

Great Elevations for Observation 94

Precision Observations at Low Elevations 96

Recapitulation of Phenomena at Observation 97

Other Important Results Affected 100

Nearly All Measured Diameters Too Larg e 101

[63] November 15, 1900.

173

VflAfl !M.i

64 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser.

Introduction.

This paper is prompted mainly by the many references
made by astronomers and astronomical publications to
" Baily's Beads" in cases of total solar eclipses; to the
"black drop" and "ligament" in transits of Mercury
and Venus over the Sun's disc; to the projection of a star
upon the bright body of the Moon at immersion or emersion
of occultation; to stars and the satellites of Jupiter seen
through the limb or body of that planet at occultation ; to
planets surrounded by an atmosphere of great depth; and
to empirical corrections to the semidiameters of the Sun
and Moon at meridian transits.

We have lately examined the reports of the observers of
the transits of Venus in 1874 and 1882 ; of the transits of
Mercury since 1878, and the reports of the total solar
eclipses since i860; and we are amazed at the crude and
apparently indigested explanations given for certain appear-
ances attending the phenomena. There would seem to
have been a desire to seek explanations in some obscure
cause or causes — such as the poor character of the objective
used, the lack of sharpness in the limbs of the planets
(owing to their possessing an atmosphere), diffraction,
telescopic and occular irradiation, astigmatism, "sympathetic
attraction," etc.

These explanations are very unsatisfactory, and the
experienced observer, who has observed occultations of
stars by the Moon or by Jupiter, meridian transits of the
Moon, transits of Mercury and Venus across the Sun's
disc, and total solar eclipses in a serene atmosphere, knows
that the outlines of the planets, the Moon, and the Sun are
remarkably sharp and steady, even under high magnifying
power; that the observations made under such conditions
are satisfactory; and that there are no extraordinary phe-
nomena present.

In such an atmosphere the spurious disc of a star, large
or small, moves along the horizontal thread of the Meridian
Circle, or the Zenith Telescope, so steadily that the error

M.-P.-Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 65

of observation should not exceed one-tenth of a second of
arc. And the double-star observer certainly expects his
measures of distance to differ little from that amount. In
the telescope the diffraction rings of the stars are beauti-
fully delicate. On a quiet night the stars appear, to the
normal eye, fixed on the sky like minute dots of light, with-
out a sign of twinkling or irradiation, and the intensity of
the light of each star, large or small, is continuous.

The observer knows that in an atmosphere which is
remarkably unsteady, when every star to the naked eye is
dancing madly, the smallest stars have disappeared, the
smaller ones appear and disappear spasmodically, and the
larger ones near the horizon rapidly change color, and
even the planets have much irradiation and are blinking;
the star in the instrument has lost its diffraction rings, the
spurious disc is broken up, and the confused mass jumps
wildly across the micrometer thread and frequently expands
into a nebulous film many seconds in diameter. We have
measured such an apparition of Polaris forty-seven seconds
in diameter.

" Burnham has remarked that an object-glass of six
inches will one night show the companion of Sirius per-
fectly; on the next night, just as good in every respect,
so far as one can tell with the unaided eye, the largest
telescope in the world will show no more trace of the small
star than if it had been blotted out of existence." (Webb's
" Celestial Objects.")

With a serene atmosphere the transit observer is certain
of his times. With a disturbed atmosphere he can make
only an unsatisfactory estimate by eye and ear, and a still
less reliable one by the chronograph.

In the course of fifty-four years' experience as an
observer, largely in the field, we have encountered a range
of physical conditions that falls to the lot of very few. We
have made astronomical and geodetic observations in all
climates, at all seasons, from the low ocean coast to eleva-
tions reaching 12,566 feet, and with instruments of precision
of various character. Our latest astronomical observations

66 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

were made in perhaps the worst locality possible. The
peninsula of San Francisco near the Golden Gate is unique
for general unsteadiness of the atmosphere, yet marked on
rare occasions by some supreme exhibitions of quietness.

Geodetic Observations in the Sierra Nevada.

In the high Sierras of eastern California, from 7,250 to
10,430 feet elevation, after a great southeast storm with
clearing weather in the night, a strong wind from the north-
west, and a wonderfully pellucid sky and steady atmosphere
to windward that gave astonishing clearness, sharpness,
steadiness and apparent nearness to all objects, we have, at
the greater elevation, seen star-like heliotrope images with
the naked eye, at stations 120, 130, 140 and 160 miles
distant to the west and northwest, whence the cold, keen
wind was blowing. At this station, with the whole range
covered with snow, the mountain face was precipitous for
1,356 feet to the north. To the southeastward and east-
ward, over the very deep valley of the Mokelumne River,
directly under us, subject to the Sun's early warmth, the
atmosphere became excessively disturbed and the heliotrope
images in that direction were not visible to the eye at dis-
tances of forty miles. In the telescope the former images
were steady, round, and so minute that the micrometer
thread nearly covered them; the latter images were very
diffuse, irregular and unsteady, and ranged from twenty
seconds to thirty seconds in diameter. In the first case the
pointing was quickly and satisfactorily made; in the second
the pointings were unsatisfactory, even with half a dozen
occular pointings as checks.

Visibility of Jupiter's Satellites.

In astronomical work we have observed the Sun, Moon,
planets and stars when the Sun was serenely steady; and
at night not a star twinkled to the naked eye, and no irra-
diation was visible to Jupiter. Half a dozen times in our
experience we have been able to distinguish with the naked

+

(/l^lv/wiH

**^&#>*~**v

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 6*]

eye1 two of Jupiter's satellites, when close together, as one;
and at one mountain station (3,200 feet elevation) all the
members of our party witnessed a similar phenomenon.
Very curiously, that evening was slightly hazy, and very
faint stars were not visible ; but the atmosphere was remark-
ably steady. Under atmospheric condition of supreme
quietness we have placed eleven stars in the Pleiades.
^j The visibility of Jupiter's satellites by the naked eye has
generally been received with much doubt. Humboldt, in
his Cosmos, has related a well authenticated case, but we
can come to our own observers for confirmation. At the
time of the total solar eclipse of August, 1878, one of the
United States Naval Observatory observers in Colorado saw
with the naked eye three satellites of Jupiter separately —
two very distinctly and one less so. The atmosphere was
supremely serene, and the station (Idaho Springs) 7,548
feet above the sea. Two other persons, not observers, also
witnessed the phenomenon. In 1874 tne Professor and his
colleagues had doubted my seeing even two satellites close
together as one.

Conditions of Steadiness and Unsteadiness of
the Atmosphere.

Under similar atmospheric conditions the limbs of the
Sun, Moon and planets are sharply defined and remarkably
steady; the Sun-spots are beautifully distinct and their
changes of form readily followed; the irregularities of the
outline of the Moon are unmistakable; the spurious discs of
the stars are encircled by delicate diffraction rings, and
the overlapping spurious discs of very close double stars
present new forms. Under conditions of supreme steadi-
ness of the atmosphere, we believe that an observer with
keen eyesight should see, with unassisted vision, the larger
satellites of Jupiter, when favorably located, and from ten
to twelve stars in the Pleiades. Either is a severe and
unique test of steadiness and eyesight. In these periods of
great steadiness the observer aches for finer micrometers,
higher power, and more stable instruments.

1 Except with spectacles for short-sightedness.

68

CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

0 h

When the steadiness of the atmosphere begins to break,
the images of the Sun, Moon and planets exhibit occasional
tremors or shiverings; and even when this vibratory move-
ment has become nearly continuous, but not rapid, the
actual and apparently more condensed outline of the image
can be selected for measures of precision in moments of
quietude. As the unsteadiness of the atmosphere increases,
the vibration of the limbs increases in rapidity and ampli-
tude until there is finally created a blurred outline of Sun,
Moon or planet, which necessarily presents an increased
diameter of the object. The stars are similarly affected;
with the first signs of unsteadiness in the atmosphere the
diffraction rings are broken, and disappear as the unsteadi-
ness increases; the spurious disc is broken into pieces; its
march is peculiar, in presenting one fragment after another
with more or less frequency, but may continue with a waltz-
ing movement until the unsteadiness has increased and the
spurious disc is without form, dilated, jumping wildly in
every direction, and a wretched object for observation.

Similar conditions are exhibited by geodetic signals of
all classes. In a serene atmosphere poles are seen very
steady as fine, sharp images, sometimes finer than the
micrometer threads. When this atmosphere is disturbed,
tremors or shiverings carry the images to right or left one
or more diameters, and the observer selects moments of
quietude for first-class observations; finally, as the unsteadi-
ness of the atmosphere increases, the images become large,
confused, blurred and faint, and observations are difficult to
make and unsatisfactory. In the quiet atmosphere, white
poles five feet long and four and six-tenths inches in diameter,
projected upon a dark background, have been seen with the
naked eye at distances of four and one-half miles across
water. In times of great unsteadiness and equally clear
atmosphere they are difficult to see in the telescope.

Heliotrope images behave precisely as those of the stars.
On a certain line of the main triangulation of the Pacific
Coast, where plains, several mountain ridges and inter-
vening valleys were crossed by the line of sight, and where

M.-P.— Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 69

the cold ocean air sweeping through the Karquinez Strait
added ten-fold confusion, the heliotrope image from Mt.
Diablo, distant forty-five miles, was seen in the telescope
like the waving flame from the stack of a smelting furnace.
Sometimes the image exceeded sixty seconds in irregular
diameter, and its horizontal direction was very abnormal.
This condition lasted for many days. When the air became
of more uniform temperature the image was small, star-
like, and steady, and its direction normal.

In the remarkably quiet atmosphere of the mountains,
before sunset, with a pure sky for a background, we once
saw, with the two and one-eighth inches objective of the
theodolite, the observer leave his station, which was thirty-
three miles distant from Mt. Tamalpais. With a salmon-
colored sky in the west, and the atmosphere remarkably clear
and steady, Superintendent Bache observed upon the signal-
pole of Mt. Wachusett, distant sixty-one miles. The image
was nearly as dark and sharp as the cross-threads.

In winter, at great elevation, we once saw, with a three-
inch objective, just after sunrise, with a serene atmosphere,
the heliotroper projected against a snow background at a
distance of fifty-one miles. From Mt. Diablo, 3,849 feet
elevation, with the same size objective, just before sunrise,
in a perfectly quiet atmosphere, when the crest-line of the
Sierra Nevada was projected like a dark-blue saw against
the warm eastern sky, and apparently only thirty or forty
miles distant, we saw upon several mornings the observing
hut on the pinnacle of Mt. Conness. The hut was six feet
wide by seven and a half feet high, and the distance one
hundred and forty-three miles ; the width therefore sub-
tended i".6^. After the hut had been cut down to about
four feet we still made it out upon an extraordinarily clear
and steady morning. On account of the distance and great
. height of Mt. Conness (12,566 feet) the Sun rose to the

TJi/uXXausixj heliotroper about eight minutes earlier than to Mt. Diablo.
The sharpness and steadiness of Mt. Conness were supreme,
and the image of the heliotrope was like a minute star; but
so soon as the Sun's rays struck the eastern flank of Mt.

"* /fltA^

J-*'*'**

feWfc .fttUy / del Muerte

We have

70 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

Diablo the atmosphere around the observing station became
so unsteady that observations were made with difficulty and
little satisfaction.

We had a similar experience at the transit of Venus,
Station Cerro Roblero, in New Mexico, in 1882. Before
sunrise the atmosphere was so serene that we could see,
with the five-inch equatorial, the small branches of the scant
scrub growth on the dark, sharp, blue crest-line of the
Organ Mountains, twenty miles distant. Immediately after
sunrise the suddenly heated air, rising up the steep slope of
the Cerro Roblero, just in front of the observatory, caused
great waves of disturbed air to obliterate the branches of
the brush and to exhibit the border of the Sun as a remark-
ably confused outline. The direction of these waves was
very marked. At the time of the first contact of the images
of Venus and the Sun the observations were made with
difficulty and doubt, and micrometer measures were almost
impracticable. The station was about 5,676 feet above the
sea, and 1,655 ^eet above the Rio Grande and the Jornado

We have been thus prolix in order to indicate that the
principal disturbance exhibited in the telescope, as well as
to the unassisted eye, is in the immediate vicinity of the
observing station. At trigonometrical stations, in disturbed
conditions of the air, the confused and unsteady image of a
heliotrope will exhibit a movement of 5" to 15" of arc. The
observer could not see such movement if it took place
rapidly at the distant station. If the heliotrope image at
Mt. Connesswere moved bodily sideways forty-four inches,
the observer at Mt. Diablo would see it change only one
second in amplitude, supposing the movement were slow
and therefore capable of cognizance, instead of being very
rapid and not then capable of detection. This exhibition of
the locally disturbed atmosphere at the observing station can
be readily produced artificially.

At Round Top station, in the Sierra Nevada, 10,435 feet
elevation, we counted forty-seven forest fires that arose on
the flanks of the forested mountains over which we were

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 7 1

observing. The smoke gradually filled the Sacramento
Valley to a height of 10,000 feet, and obscured the outline
of the Coast Range Mountains; but we observed the helio-
tropes through this irregularly heated atmosphere with little
difficulty, and at the close of the operations the probable
error of a direction was only seven hundredths of a second
of arc.

Besides the disturbance of the outer atmosphere near the
observer, there is another source of disturbance in the air
within the tubes of the great telescopes, especially in sun-
light observations or at the sudden change of temperature
at sunset. This is visibly exhibited in the blurred, unsteady
and confused outline of the Sun and the Sun-spots projected
upon the ground-glass plate of the horizontal heliograph
tubeless telescope of forty-feet focus. In the transit of
Venus of 1882 the image of the Sun at Cerro Roblero was
projected through a forty-feet tin tube of large diameter,
and was much blurred; but this was largely corrected when
the tube was covered with a wooden roof not in contact.

Professor A. E. Douglass, of the Lowell Observatory,
has shown that the waves of the disturbed atmosphere out-
side and inside of the large telescope tubes are distinctly
visible to the naked eye when it is placed in the focus of
the objective, as in the Foucault test. And even more than
one series of such waves of disturbance at different dis-
tances and moving in different directions is not infrequent.

It is well known that to obtain the best results with the
great telescopes it is imperative that the air inside the dome
and inside the telescope tube shall have the temperature of
the outside atmosphere.

At Arequipa, Peru, the Harvard Observatory was situated
close to the valley of a mountain stream or arroyo, down
which, on clear nights, a swift stream of cold air descends.
Whenever this cold stream overflowed the banks and envel-
oped the observatory, and rose to the height of the objective,
the seeing was immediately ruined for the rest of the night.
In such circumstances, if the eye-piece of the telescope was
removed and the eye placed in the focus, fine parallel, dark

72 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3d Ser.

lines could be seen moving swiftly lengthwise across the
illuminated lens in the direction of the wind.1

At San Francisco, in the strong "wind gap" of the
Golden Gate, it sometimes happens at sunset, after a calm,
moderate day, that star images are disc-like and steady; in
a few minutes the westerly wind rises and brings in the fog-
chilled air from the ocean, with flying patches of fog over-
head, and the star images at once become diffused and
nebulous, with an unsteadiness reaching an amplitude of
5" to 10" of arc.

Large volumes of air, each of different but uniform tem-
perature, projected across a line of sight at a great distance
from the observer, do not necessarily produce abnormal
images of a star or heliotrope; they seem to act as prisms,
and deflect the line of light so as to exhibit abnormal hori-
zontal or vertical refraction without any marked unsteadi-
ness. Along the face of a great mountain wall we have
measured a slow change of azimuth of a signal amounting
to 65" of arc, due to the gradual rising of the morning tem-
Cavvw I peratureof the air immediately adjacent to the eastern rocky

Cr ^ face of Mt. Constitution.

There are so many conditions conspiring to a disturbed
atmosphere that it necessarily acts in various ways difficult
to predict. Where there are regular, irregular or confused
atmospheric waves moving at different distances from the
objective, and in different directions, they necessarily give
different images of the star at different foci, and at each
change they bodily shift the image near the focus, and also
change its form. Professor Douglass' observations are very
instructive in this matter.

When volumes of unusually heated air pass over the
objective they may act as a species of air lens which sud-
denly and irregularly spreads out the image of the star at
the focus; but no change of focus will bring the image to
condensation. This we first experienced at Point Concep-
cion in 1850, where the perceptibly hot volumes of air from

1 Douglass, " The American Meteorological Journal," March, 1895, p. 395.

^^^^(7^^^*^. A^+^£r/&~» Ay*4.

M.-P.— Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 73

the higher gulches were driven over the observatory by an
incoming cold north wind from the Sierra Santa Inez.

It is sometimes reported that stars become very unsteady
in a strong wind; yet we have determined micrometer
values by observations upon a close circumpolar star during
a fierce " norther." In geodetic observations between Mt.
Diablo and Mt. Conness, on a line of 143 miles across the
Sacramento Valley, we witnessed a strong norther blowing
down the valley and carrying vast quantities of dust across
the line of sight; but the wind did not reach the observing
station on the first day, when the heliotrope image was
fairly good, but was moved bodily by horizontal refraction
two or more seconds of arc from the normal direction.

Observed Occultations of Red Stars at the
Bright Limb of the Moon.

Referring now more particularly to the details of some
astronomical observations, we give a few instances from our
experience and that of others.

On September 18, 1848, about seven and one-half hours
in the morning, we observed the occultation of a Tauri by
the bright limb of the Moon, using a telescope of probably
two and one-half inches aperture and moderate power.
The red star touched the bright limb, and we noted the
time mentally, because it did not instantly disappear. We
continued the watch until the star was unmistakably within
the apparent limb, when it disappeared instantly, about two
and a half seconds after the first apparent contact. We at
once submitted the case to our chief, R. H. Fauntleroy,
who said the disappearance was the true time of the occul-
tation, but he gave no explanation of the phenomenon. We
then submitted the observation to the Superintendent of the
United States Coast Survey, Professor A. D. Bache, who
wrote to the Royal Astronomer (Airy) on the subject.
The reply was that he personally had never observed the
phenomenon, but there were similar cases on record. He
gave no explanation.

no /> v^ * Hj*Ja ^^/Miu, i^U^U^ y^<^ys*^n*

74 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3d Ser.

In the trigonometrical survey of Admiralty Inlet and
Puget Sound, during the approach of a southeast storm
which finally brought up dense clouds, a heavy squall and
rain, we observed a Scorpii (April 22, 1856,) with a three-
inch Fraunhofer telescope and astronomical eye-piece, power
about 70. The disappearance of the star was behind the
bright limb, and the record states that " the time denotes
the instant of the disappearance of the star after it. had been
projected upon the body of the Moon for about two and one-
half seconds, certainly not less. The ruddy color of the star
showed distinctly upon the body of the Moon, and the
instant of disappearance was as accurately noted as if it
had disappeared behind the dark limb. Having once before
observed the same phenomenon, we were fully prepared
for it."

In November, 1886, we presented a paper to the Cali-
fornia Academy of Sciences upon the occultation of a Tauri
on the twelfth of November. Three instruments were used
at the disappearance: two three-inch Fraunhofers, power
100, and a two-inch, power 55. The atmosphere was clear
and moderately steady, the stars showed signs of unsteadi-
ness, and the Moon's border appeared reasonably sharp.
The telescopes were not large enough for such observa-
tions, but an intervening house prevented the use of the six
and four-tenths inches Equatorial.

The star was visible to the unassisted eye to within four
minutes of contact. When it was a few seconds from the
Moon's bright limb the star became invisible in the two-inch
telescope. In the three-inch Fraunhofers the star did not
disappear when it touched the apparent limb of the Moon,
but continued to move upon the disc until it was fully one
and one-half times the diameter of its spurious disc on the
Moon, when it disappeared with the instantaneity of such
phenomena. The star grew somewhat difficult to see after
it entered upon the factitious limb, but its reddish hue left
no doubt in the minds of the two observers, who differed
0.07 second in the time of its disappearance. They esti-
mated that it was three seconds of time on the limb. At

&4h#

/flu <*c£^£> V^<

M.-P.— Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 75

emersion from behind the narrow, dark edge of the Moon
(which was one day past the full) the 6.4-inch Equatorial
noted the reappearance 0.17 second earlier than the two
three-inch telescopes.

At the Chabot Observatory, thirty-six seconds of time
east of the Davidson Observatory, the observer used the
Clark eight-inch Equatorial for the same occultation. The
atmospheric conditions were nearly the same as mentioned.
The star advanced upon the apparent limb of the Moon,
but not so far as above mentioned, and some of the apparent
rays of the star projected outside the Moon's border.

On March 29, 1887, we observed the immersion of
a Tauri at the dark limb, and the emersion at the bright
limb; the former by daylight, and the ash-gray limb not
visible. The disappearance was instantaneous. At reap-
pearance the Moon's limb was blurred and very unsteady,
and the star reappeared upon this blurred bright image
inside of the outermost limit, which was very fuzzy and
decreasing in brightness outward. The star was not so
bright as anticipated, but its almost sparkling red color left
no doubt whatever in our mind as to the nearest tenth of a
second of the epoch. The star left the limb in two and one-
half or three seconds, when it appeared much brighter. At
emersion a power of 250 was employed- ♦

We do not mention the hundred and more occultations
which we have observed when the above phenomenon was
not present, or when the smallness and whiteness of the star
may have prevented its detection.

Other Observed Occultations of the Stars by

the Moon, and of Stars and Satellites

by Jupiter.

In 1890, the Astronomer Royal called the attention of the
Royal Astronomical Society to the occultations of f Tauri
(3.0 magn.) that presented matter of ''interest" to
observers and astronomers. On September 16, 1889, Mr.
Turner, of the Greenwich Observatory, observed the occul-
tation of the same star by the bright limb of the Moon with

76 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

the Lassell Reflector of thirty-four inches, and Mr. Lewis
with a refractor of three and three-fourths inches. Mr.
Turner noted: "No projection; disappeared instantaneously
at bright limb." Mr. Lewis noted that " the star touched the
limb of the Moon five seconds before the observation, and
was slightly inside the limb." It appeared " as a brilliant
spot in the Moon, and disappeared suddenly at the time
given above." In the same number of the " Observatory "
where this report is made Mr. Ranyard says that " in the
case of Jupiter there are instances * * * where one or
two stars have apparently been seen through the limb of the
planet. There are such a number of these observations that
we cannot doubt the planets have not a sharp limb; they
seem to be surrounded by an atmosphere of great depth, or
rather a gaseous envelope, in which clouds or dusty matter
float in irregular masses." This is an extraordinary state-
ment, and, in our judgment, based upon a misconception of
the cause of these phenomena.

The outline of any cloud system at such enormous dis-
tances from the Earth as Mars, Jupiter and Saturn would
subtend so minute an angle that the outline of the planet
would necessarily appear sharp, even if the assumed cloud
were irregularly distributed. The Sun's outline appears
remarkably sharp when our atmosphere is steady, and yet
we know that extraordinary disturbances of the solar
surface, far beyond our experience, are constantly taking
place.

On May 10, 1895, Mr. John Tebbutt observed the occul-
tation of Antares at Windsor, New South Wales. The
steadiness and definition of objects were reported very sat-
isfactory. The instrument was an eight-inch objective,
with a magn. power of 74, and "Antares, quite brilliant to
the last moment, was closely watched till it came into con-
tact with the bright limb; but then, instead of instantly
disappearing, it seemed to cut its way into the disc during
two or three seconds, and vanished instantaneously. The
reappearance of Antares (62 1-3 minutes) later at the dark
limb was remarkably instantaneous." It should be noted

M.-P.-Voi.. I.] DAVIDSON— APPARENT PROJECTION, ETC. 77

however, that the observer was not looking at the spot
where the star reappeared, and thereby lost the earlier
reappearance of the well-known companion; and at any
rate he could not have seen the star reappear with sudden
brightness unless the dark limb was undisturbed. The
same occultation was observed at Waverly, Sydney, by
Walter F. Gale, under fairly good conditions and thick
haze. With a six and a quarter-inch objective and magn.
power 140 "the disappearance was instantaneous at a
notch formed in the Moon's limb by two peaks." This
description of the point of ingress indicates that the limb
of the Moon was steady and sharp.

The same occultation was also observed at Warrickville,
near Sydney, by C. J. Merfield, with a seven and a half-
inch reflector, with power 170. "The definition was fair,
although hazy at the time of disappearance, which was not
instantaneous, as is usual."

Professor Young, in his " General Astronomy " (p. 246),
refers to the projection of a star on the Moon's dark limb
in the following paragraph: " In some cases observers have
reported that a star, instead of disappearing instantaneously
when struck by the Moon's limb (faintly visible by earth-
shine), has appeared to cling to the limb for a second or
two before vanishing, and in a few instances they have even
reported it as having reappeared and disappeared a second
time, as if it had been for a moment visible through a rift
in the Moon's crust. Some of these anomalous phenomena
have been explained by the subsequent discovery that the star
was double, or had a faint companion." No further explana-
tion is attempted. We have purposely watched stars
occulted at the ash-gray limb of the Moon, but have never
been able to see a sufficiently bright and defined limb to
note whether it was invaded by the star before its sudden
disappearance.

In the publications of the Astronomical Society of the
Pacific for November 30, 1889, Professor Barnard, of the
Lick Observatory, says that " at a number of occultations
of the satellites [of Jupiter] I watched carefully for any
evidences of their being seen through the edges of the

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/fat , /?}^rt6' ■

78 'CALIFORNIA ACADEMY OF SCIENCES [Proc. 3d Ser.

planet, but saw nothing of the kind. Professor Holden
informs me, however, that with the thirty-six-inch Equa-
torial the whole disc of a satellite has been visible within
the planet's atmosphere at every occultation he has ob-
served." In the "Astronomical Journal," volume VIII,
page 64, there is a record by these observers of the occul-
tation of 47 Libras (6.4 magn.) by Jupiter, on June 9, 1888.
With the thirty-six-inch objective and power 672, the images
were below the average of good seeing. Director Holden
reports that "the star entered the limb and was seen
bisected" at im 30s after it had touched the outer edge.
" The entire image of the star was seen, inside as well as
outside the limb, being easily distinguishable from the
planet's surface by its brilliancy and peculiar color." In
2m g£8 << the star was entirely inside the limb, but still vis-
ible. For the next ten seconds the star was alternately
visible and invisible, and the planet's limb was quite un-
steady." In 2m 51s from the star's first touching the limb it
was certainly gone. Professor Barnard observed the same
occultation with the twelve-inch objective, reduced to eight
inches because " the images were too unsteady with the
full aperture" (magn. power 240). He noted the time of
" contact of the following edge of the star with the preced-
ing limb " of Jupiter. At im 12s after )/Nhe star was partly on
the limb; at im 51s the star was seen by glimpses to that
epoch, and three seconds later it had certainly disappeared
when it had advanced about three-quarters of its disc within
the limb. On the usual scale of seeing, the conditions were
below the average; but even when the star had encroached
on the limb the disc was small, round and bright, and as
clearly defined as that of a satellite entering transit. No
certain diminution of its light was observed.

Professor Barnard makes no allusion to the color of the
star, and as its spectrum is recorded as that of the first type,
there seems no reason why the first mentioned observer
should attribute a " peculiar color " to it.

Under date of April, 1894, Professor Barnard thinks
" astronomers should reject the idea that the satellites of
Jupiter can be seen through his limb"; since, under good

M.-P.— Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 79

conditions, with the Lick thirty-six-inch objective, " the limb
of Jupiter has appeared perfectly opaque, as in all my previous
observations with smaller telescopes." But he makes no
reference to the character of the factitious limb of the
planet in unfavorable conditions of the atmosphere.

At Melbourne, September 14, 1879, the occultation of 64
Aquarii (6.9 magn.) by Jupiter was watched in the great
reflector. The star was thirty-five seconds in disappearing,
and remained visible ten seconds longer as a speck of light
seen through ground glass. This speck " also disappeared
gradually." Proctor says the observers personally assured
him the phenomenon was not due to irradiation.

In 1878, at the Adelaide Observatory, two observers
(Messrs. Todd and Ringwood) saw the occultations of
satellites I, II and III upon certain occasions "within the
disc of the planet " at disappearance, and once at reappear-
ance. " In every other case the occultation was perfect at
the limb." This latter sentence was not quoted by Proctor.

Dr. E. C. Pickering, Director of Harvard Observatory,
has described an occultation of D. M., 230 1087 (7.3 magn.),
by Jupiter, April 14, 1883. He used the fifteen-inch
objective, with achromatic eye-piece, power 400. " For a
period of about ten minutes after disappearance was
observed, the star is recorded as 'suspected.' The record
states that for some time previous to final disappearance
' the star alternately disappeared and reappeared without
cause; seeing pretty good and uniform throughout.' At
the reappearance, the limb of the planet was watched con-
tinuously for forty-nine seconds, at the beginning of which
the star was not visible, and at the end of which it was dis-
tinctly seen, remaining visible thereafter."

In our experience in observing Jupiter, we have never
seen the border sharply defined when using powers of 500,
but atmospheric conditions have never been supremely
good at the times of observation. In observing the occul-
tation of Jupiter's satellites through an unsteady atmosphere,
our fellow-observer, with a smaller telescope and objective

( 2 ) November 19, 1900.

J

80 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

of two and one-half or three inches, lost the satellite much
sooner than we observed its disappearance in the 6.4-inch
Clark Equatorial, when we followed it into the factitious
limb ; and with a disturbed atmosphere this advantage should
always pertain to the larger objective and higher power.

Numerous observers might be quoted who have seen
stars and satellites projected within the apparent limb of
Jupiter at occultation. Some of the details are rather
startling, and perhaps the most notable is that by Admiral
Smyth, in 1828, supported in part by two other observers
at stations a few miles distant. The satellite II remained
for some minutes projected upon the apparent edge of the
planet, and thirteen minutes afterwards it was outside of the
disc; then it remained visible four minutes and " suddenly
vanished." If the observers had been experts, observing
with instrumental means adequate to give sufficient light,
definition, and size to the images — noting the times with
accuracy, and fully recognizing the effect of the locally
disturbed atmosphere — we are satisfied that some satisfac-
tory explanation would have been reached.

Such details as these, and others upon different occa-
sions, are rather astonishing. Such remarkable conditions
must be due to imperfect instrumentation, to inexperience,
to weariness or lack of quick and positive accommodation
of the eye, to erroneous estimates of small intervals of time,
to extraordinarily abnormal conditions of the atmosphere, to
intervals of unconscious cerebration, or to gross mistakes.
It was from the abnormal observations at Melbourne that
Proctor undertook to deduce the depth of Jupiter's atmo-
sphere, and concluded that the star 64 Aquarii was seen
through a range of 10,000 miles of atmosphere below a
depth of 300 miles.

We may here mention incidentally that for many years we
have been on the outlook for Saturn and his ring system to
occult some star of moderate magnitude; but the phenome-
non has not occurred in our experience. We were anxious to
learn how the ring system would treat the star.

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 8l

Similar Phenomena at Total Solar Eclipses. '

Many reports of observations of total solar eclipses and
transits of Venus and of Mercury over the Sun's disc are
painfully discordant, misleading and uninstructive. They
betray inexperience and lack of knowledge of the local
physical disabilities that may present themselves. It would
almost appear as if those who had the least experience were
the most authoritative in their opinions. And certainly,
when the results are published, there is little or no discrim-
ination made in assigning proper weights to them. Some
observers make no mention of their defective eyesight, and
♦ we have personally known observers, quoted as authority,
that should never have been trusted to attempt any observa-
tion of precision.

The elaborate reports of the total solar eclipse of 1869.
in the United States, by many observers in different locali-
ties, are quite instructive in the unconscious exhibition of
contradictory statements and inappropriate adjectives. Two
observers at the same station reported directly opposite
conditions of the steadiness of the limbs of the Sun and
Moon. One said they were defined with the utmost clear-
ness; another sent us drawings to explain his statement that
they were "boiling excessively." And yet the observer
who reported steadiness was in doubt twelve seconds of
the time of contact, and took the mean of his limits of
doubt. And it is evident that the many observers had dif-
ferent conceptions of what constituted the phenomenon of
" Baily's beads." Some looked for, and evidently ex-
pected, the exhibition as a necessary and normal physical
condition. The tabulated descriptions of the phenomena
at totality range wildly; we find recorded: "no trace of
Baily's beads;" "the phenomenon of Baily's beads lasted
but a few moments;" "Baily's beads beautifully distinct
and spreading over an arc of 300;" "Baily's beads were
seen fifteen seconds or more; they were long and thin,
moved with the moon, and were unquestionably the effect
of the irregularities of the Moon's contour exaggerated by

82 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

irradiation." One observer has given detailed drawings of
the form of the "beads," and exhibits them with mechan-
ical precision and hardness; as a matter of fact he was not
a draughtsman nor a skilled observer, and yet his name
carried great weight. Ci, v Oi * VoV-J^

Similar Phenomena at Transits of Venus and Mer-
cury across the Sun's Disc

In our observations of the transit of Venus, December
1882, at Cerro Roblero, New Mexico, 5,676 feet above the
sea, and 1,655 ^eet above the Rio Grande del Norte and the
great arid plain of the Jornado del Muerte, there were,
towards the close, occasional tremors or shiverings of the
borders of the discs of the Sun and planet from slight
tremulousness of our atmosphere; but the eye was not con-
fused, as would have been the case if the discs had con-
tinued ill-defined and unsteady by excessive vibration. In
the first case the eye could and did select its opportunity
for micrometric measurements. But at this very period the
assistant astronomer in charge of the photographic expo-
sures reported the results unfavorably affected by this
slight unsteadiness. It was a case where the eye selected
a moment of steadiness, but the mechanical movement of
the photographic shutter permitted no selection. The un-
steadiness of the atmosphere at the time between the third
and fourth contacts permitted a beautiful and unique exhi-
bition of the fine, white, faint, crescent of coronal light
apparently surrounding part of the disc of Venus as an illu-
minated atmosphere. The crescent was long, extremely
thin, white, and as fine, sharp and regular as if cut by the
finest graver; and we watched it die away in excessive mi-
nuteness. We made drawings of the exhibition. Another
observer of this beautiful phase of the transit at a different
station saw a similar phenomenon "shortly after the first
external contact, when the limb of the planet was 'boiling,"
and "it was difficult to be certain whether it lay within or
without the planet's contour * * * but it certainly ap-
peared to extend at any rate within the circumference; and

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 83

indeed it presented the appearance of 'Baily's beads' at
the time of a total solar eclipse. Its whole depth was one-
fourth of the planetary radius"; and the published drawing
showed the broader part of the coronal light partly inside
the planet's circumference when the planet was half on the
Sun's disc. Astr. Nach. 2,481. Here we have exhibitions
of the same or a similar phenomenon at the same transit
under widely different atmospheric conditions; and the first
is surely to be taken as better representing the true or nor-
mal phase of the phenomenon, and the latter is to be
reckoned as abnormal and misunderstood.

This phenomenon of the so-called " atmosphere of
Venus," though not connected with the disturbance of our
atmosphere, is thus seen to be affected by it just as much as
the outline of Sun or planet. Mouchez has called it the
"pale aureole * * * to be attributed in part to the at-
mosphere of the Sun rendered visible by contrast, and also
in part by the atmosphere of Venus." Heraud and Bonify
designated it as the "filet lumineuse;" and Airy says this
"penumbra must, of course, be considered a part of
Venus;" and elsewhere he says "the partial illumination of
the atmosphere of Venus introduces difficulties of observa-
tion" etc. One observer says that this ring of light around
Venus enabled him to see the planet twenty-four minutes
before contact. One of our astronomers who had wit-
nessed the transits of Venus of 1874 a°d z882 asserts that
"there seems to be absolutely no way of escaping from a
new difficulty — the planet's atmosphere causes it to be sur-
rounded by a luminous ring — as it enters upon the Sun's
disc, and thus renders the time of contact uncertain by at
least five or six seconds." Another admits "the disturb-
ance of the image by irregular refraction in the Earth's
atmosphere," but declares that "we must look to the chief
cause of the great discordances" of the times of contact of
Venus and the Sun, "in the atmosphere which surrounds
Venus." And he further says, that "the cruel manner in
which all the phenomenon are modified by the atmosphere
of Venus is not easy to explain." Another declares "the

84 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

planet was surrounded by a ring of light, as bright as the
Sun, that pushed the edge away from it; and no actual con-
tact could be observed." And yet there was no black drop,
no ligament and no distortion reported by the observer.

It is admitted that there is no atmosphere enveloping
the Moon, and yet Royal Astronomer Stone at the Cape of
Good Hope observed the outline of the Moon projected
against the Sun's corona five minutes and eight seconds
before the total phase of a solar eclipse. We reported a very
faint exhibition of this phenomenon January nth, 1880.
If the corona of the Sun is the visible arc of illumination in
this case through contrast, we are entirely justified in as-
suming it to be the agent of illumination at the circumfer-
ence of Venus; and on account of the irregularity and
inequality of the brightest parts of the corona, it may be
seen at one part of the planet's circumference and not at
another, or it may be seen all around the planet. A critical
examination of the reports clearly demonstrates that this
faint illumination on the border of the dark body of the
planet is seen as (1) part of a circumference; (2) at dif-
ferent parts of the circumference; (3) of varying breadth
at different points of its length by the same observer; (4)
as wholly encircling the planet. If it were the atmosphere
of Venus it would be uniform and continuous, unless a
Venus cloud system interfered irregularly. Professor
Young says (p. 159) that at a total solar eclipse there is
"not any ring of light running out on the edge of the Moon
like that which encircles the disc of Venus at the time of
the transit."

With a slight digression, we mention one or two vagaries
of observers of the transits of Mercury.

At one of the recent transits where the observers were
side by side, one reported the "definition of Mercury sharp
and steady," and the "limb of the Sun undulating," as if the
two objects were unequally affected. The other observer
described the "limbs of the Sun and Mercury loosely con-
nected by a patch of haziness oscillating between them;"
and at another phase the "undulations of the Sun's limb

M.-P— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 85

occasionally reached Mercury;" leaving the inference that
the planet was steady. At yet another station the "planet
appeared with rippling fiery and black balls chasing each
other between the planet and the Sun." In the transit of
1878 one observer "was surprised at the absence of glare
and tremor; and the edge of the Sun's disc was clearly de-
fined and remarkably steady with the exception of a few
notches and scratches."

We had been fortunate in our observations of the two
transits of Venus, and of several transits of Mercury, never
to have seen an exhibition of "black drop" or "ligament,"
except at the transit of Mercury in 1891, when the atmos-
pheric conditions of San Francisco were very unfavorable
although the sky was clear; the limbs of the Sun and
planet were both spurious or factitious on account of exces-
sive vibration; and the observation was necessarily difficult
and doubtful. Under such adverse conditions the observa-
tion may be very wild, or it may turn out very good; but
certainly the observer cannot assign much weight to the
epoch noted.

Instructions for Observing the Transit of Venus.

In re-examining the large number of reports of the transits
of Venus of 1874 an(^ 1882, we have gathered forty or
fifty expressions of the observers in their endeavors to de-
scribe the phenomenon usually known as "black drop,"
"ligament," or "filament."

This need hardly be wondered at. The British Instruc-
tions of 1882 assert that the phenomena seen by most ob-
servers near the time of contact are of a complex character,
and extend over considerable intervals of time. These in-
structions surely conveyed the impression that these complex
phenomena were the normal physical conditions. They
added confusion to previous descriptions by mention of the
"light of the cusps;" and evidently regarded the exhibition
of haze, or ligament, as "the glimmering of the light of the
'aureole,' 'penumbra', or 'sunlight' refracted through the

86 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

atmosphere of Venus across the dark space between the
cusps." And further, if a "pure geometrical contact" (which
is the only line that can be given for contact) is frustrated by
haze, shadow, ligament, or black drop, then the last marked
discontinuity of the illumination of the Sun near the point of
contact is the epoch to be recorded if it "is distinctly recog-
nized as independent of mere atmospheric tremor."

The American Instructions were mainly based upon the
appearances of 1769; and declared the "atmosphere of
Venus clearly illuminated." They refer to the difficulty of
observing the moment of tangency on account of imperfec-
tion of vision, irradiation produced by the Earth's atmos-
phere, and imperfections in the telescope; and then
mention the difficulties to be expected by atmospheric
undulation.

One of our eminent astronomers accepts the explanation
of Lalande, that the phenomenon of "black drop," etc., is
caused by irradiation at the bright object; and that this irra-
diation arises from a number of causes, imperfections of the
eye, imperfections of the telescope, and the softening effect
of the atmosphere upon a celestial object when seen near
the horizon. He likens it to a narrow and less bright false
edge around the bright object; and says that this band will
appear narrower the better the telescope and the steadier
the atmosphere. He furthermore says that in the observa-
tions of the transit of Venus of 1874, with the improved
instruments, very few of the experienced observers no-
ticed any distortion at all. Later on he adds that "in the
varied forms presented 'when the air is not still,' we recog-
nize all the peculiar appearances described by the observers
of 1769." In 1874 tne Mexican observers at Yokohama
reported to us a decided exhibition of "black drop."

Colonel Tennant has remarked of the black drop phenom-
enon, " there is no doubt in my mind that the outer part of
the Sun is never free from the result of outstanding astigma-
tism." We have astigmatism of the human eye, and astig-
matism of the telescopic objective and ocular, but we do
not understand to which he applies the term, if to either of

M.-P.— Vol. L] DAVIDSON— APPARENT PROJECTION, ETC. 87

them. And one of our best astronomers, in reviewing the
physical and other conditions of the 1761, 1769 and 1874
transits, summed up his deductions by saying that " the
black drop, and the atmospheres of Venus and the Earth,
had again produced a series of complicated phenomena
extending over many seconds of time."

Curious Descriptions of the Phenomena.

With much more speculation of similar import we should
be prepared for the crude descriptions and attempted ex-
planations given by the observers. They range from
a 'Chinaman's hat' to a 'pear-shaped planet,' and are even
stretched out to a 'gourd shape.' The descriptions and ex-
planations by the same observer are sometimes curiously
contradictory or vague, as: 'the limbs were boiling vio-
lently,' 'yet quite sharp and doubt only three or four
seconds;' there was 'black drop' but 'no distortion;' 'no
distortion' yet the 'limbs of the Sun and Moon were spin-
ning;' ' interference lines;' ' Venus serrated;' 'the Sun's
limb had lost its sharpness from the overlapping of Venus'
atmosphere;' the ' shadow of contact;' from the overlap-
ping of the two 'penumbras of imperfect definition;' and
most remarkable and incomprehensible, ' the sympathetic
attraction or assimilation ' of the limbs of the Sun and the
planet at the second contact but no 'mutual attraction' at the
third.

In Egypt, in 1882, one observer paid particular attention
to detect the black drop and could not see it; another ob-
server at the same locality observed the black drop. The
imagination would seem to have played a part in the obser-
vations. One observer saw Venus twenty-four minutes
before contact; and near the first contact he saw "a dis-
tinct cone of shadow thrown away into space;" and his
drawing is stronger and stranger than his description. In
1874 *he chief astronomer of one of the foreign expeditions
declared to us that the phenomenon was simply and solely a
case of diffraction.

88 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

Errors of Observation.

It is therefore astonishing to note that under such adverse
conditions for accurate observation the times of contacts
are noted to tenths of seconds. As two seconds of time
mark the apparent relative movement of the bodies about
one-tenth of a second of arc, we need not wonder that some
of the observers were not sure of the minute; and in one
case an error of three minutes could not be fixed. This
occasional and accidental distortion of the planet's disc
in such transits had called for the determination of the
epoch of the geometrical contact of the limbs of the images
of the two bodies and for other appearances ; but under
such unfavorable conditions the judgment of the best ob-
server may reasonably have been at fault, and his recorded
epoch of contact very wild. This is shown by many illus-
trations of the distortion where the outlines of the "black
drop" are drawn with a hardness and positiveness that are
certainly uninstructive, and unsuggestive of the cause.

The difficulty of observation is made painfully manifest
in the experiments at Washington early in 1874, when most
of the observers practised upon an artificial transit of
Venus to determine the times of the four contacts. The
observations range from four seconds before the first con-
tact to twenty-eight seconds after; and even at the second
and third contacts, from twenty-one seconds before to sev-
enteen seconds after the epochs. These extraordinary per-
sonal equations must surely have been due in part to inex-
perience, and mainly to the disturbed conditions of the at-
mosphere. And yet the mean value of a series of such
observations was applied to the actual observations in
the field upon the Sun and planet, to reduce them to
a systematic series. Had these preliminary observations
been made in a serene atmosphere, the apparent per-
sonal equations would have been brought to normal and
reasonable limits, notwithstanding one observer declares
that "the optical edge of a bright body is not, and in the
nature of things, cannot be, absolutely sharp in the eye or
in the telescope." The great discrepancies arose in large

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 89

measure from the unsteadiness of the short line of inter-
vening atmosphere near the ground; and we then related
our experience at great elevations and over heated plains,
and urged the occupation of elevated and isolated stations
as the most effective means of avoiding an unsteady atmos-
phere in such costly and important observations.

Observations of Solar Eclipses under Different
Atmospheric Conditions.

Professor Dr. Schaeberle of the Lick Observatory ob-
served the total solar eclipse of October 9, 1893, at 6,600
feet elevation, on the western flank of the Andes in Chili.
He was eminently successful in all his observations on ac-
count of the extreme steadiness of the atmosphere. The
Sun's image, projected by the six-inch equatorial upon a
white cardboard, looked "more like an engraving than an
optical image." Aubertin, at the same station, said that
during the eclipse "the atmosphere was absolutely pellu-
cid"; he had had experience at Gibraltar in 1870.

Our experience in observing the above eclipse at San
Francisco as a partial phase was the reverse of Professor
Schaeberle's, but very instructive. We condense the report.
The images of the Sun, and the micrometer thread were
projected by the 6.4-inch equatorial upon a white sheet of
paper. The apparent diameter of the Sun's image was
about twenty-two inches. The atmosphere was unsteady,
and the border of the Sun was confused and blurred, and
lacked the solid or consistent brightness of the^disc. This
factitious border was about half a millimeter broad at the
least. The solar spots were confused and their details were
not distinguishable. As the time of first contact approached
we watched the predicted point of contact, not for the first
indentation of the Sun's disc, but for the first commingling
or overlapping, so to speak, of the factitious image of the
Moon's border with the factitious image of the Sun's border.
There first appeared a very faint darkening of the confused
and apparently expanded border of the Sun's limb. We
noticed it when it was about three millimeters long; the

90 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

disturbance of the limb was very great. The dark com-
mingling or overlapping increased in length, breadth, and
darkness; and at last changed to a line of blackness when
the actual limb of the Moon's image touched upon the Sun's
condensed, brighter and actual border. There was no
hesitation in noting the time of this contact by several
observers. At the time of the last contact, four of the
observers watched the phenomenon. The borders of the
two discs were more unsteady than at the first contact, and
the reversed order of the first contact-appearances took
place. A great mountain range of the Moon was the last
to disappear. The two exhibitions were very instructive.
At the total solar eclipse of January 11, 1880, on the
Sierra Santa Lucia, 6,100 feet above the sea, we observed
the phenomenon under peculiar circumstances fully detailed
in the official report. The atmosphere was beautifully
serene after a prolonged and terrific storm of wind and
snow. The limb of the Sun was not absolutely steady but
exhibited occasional tremors or shiverings, and there was no
disturbance of the limb at first contact. " The cusps were
very sharp and clear, and whenever a tremor occurred on
account of any slight atmospheric disturbance these cusps
were apparently doubled." (At Oakland where the atmos-
phere was much disturbed the cusps appeared confused and
blunted.) As totality approached, the crescent of the Sun
was remarkably long and narrow on account of the slight
difference of the apparent semi-diameters of the Sun
(16' 18". 1) and the Moon (16' 23" .5). The last line of the
crescent was 300 to 400 long before it broke. It exhibited
no distortion from atmospheric disturbances except an occa-
sional tremor or shivering. The cusps, before the crescent
was reduced to a line, were remarkably sharp, curved points,
as if cut by the finest graver. The breaking of this last
thin line of sunlight was occasioned by the intrusion of the
lunar mountains and the inequalities of outline. It pre-
sented the appearance of a line of bright dots and dashes,
and black spaces. There was no wavy motion to interfere
with this exhibition; whenever a bright spot or a line

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 91

disappeared it was instantaneous, and gone forever. The
moving dots of the " Baily's beads" were absolutely
wanting. At the last contact of the eclipse the atmospheric
conditions were wholly changed. The atmosphere was in
a remarkable state of undulation and the limbs of the Moon
and Sun were moving in great rapid waves, and the obser-
vation was unsatisfactory. The Sun set below the ocean
horizon about ten minutes later and we should have ex-
pected a quiet atmosphere, but the warm Sun rays heating
the air upon the steep, snow-covered ocean-side of the
mountain mass caused rapid evaporation of the snow, and
irregularly heated currents of air to flow over the station.
At this eclipse we called attention to the less darkness of the
sky adjacent to the advancing body of the Moon, due to
contrast with coronal effects.

At the solar eclipse of October 30, 1883, we projected
the images of the Sun and Moon upon a sheet of white
paper, and studied the disturbed outline of both objects.
The greatest obscuration occurred just before sunset, when
the Moon was half way over the Sun's disc. Owing to the
extreme refraction at this zenith distance, and the "exceed-
ingly great boiling" of the borders of the two images, the
"distortion of the cusps was striking and peculiar."

In contrast with these two exhibitions of great atmospheric
unsteadiness, we introduce an experience approaching that
of Professor Schaeberle's above mentioned. ,

We observed the total solar eclipse of August 7, 1879, on
the Chilkaht River in Alaska, near the end of heavy, cloudy
weather, when the atmosphere had become clear and steady.
At the first contact "the limb of the Moon was very sharply
defined, and the outline of the Sun was very steady and
sharp." Just before totality "the crescent was very beauti-
ful to the unassisted eye, and in the telescope." In the
three-inch Fraunhofer, with moderate power, the "borders
of the Sun and Moon were remarkably steady and very
sharply defined. In twenty-four years of practice in observ-
ing we have rarely, if ever, seen them under such favor-
able circumstances. We observed them without any shade ,

92 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Skr.

and followed the Sun's border to the instant of disappear-
ance. The bright, long, narrow crescent was sharply and
regularly defined throughout; the •extremities were clear-
cut and pointed. As the width and length of the crescent
decreased, this sharpness of outline and regularity of form
were maintained until it became a fine line of living white j
and, shortening rapidly, it disappeared as a very short, fine,
distinct line, and not as a star at its disappearance. There
was no breaking of this line into points or heads; no wave
motion along it; no disturbance whatever of continuity or
regularity of form . ' '

"A feature of the phenomenon of totality was the vivid
impression that the dark body of the Moon stood out clearly
and unmistakably in relief in the space between the
observer and the coronal brightness around the obscured
body of the Sun, and did not lie flat and upon it, as in a
picture." This perspective effect resulted from the seren-
ity of the atmosphere, the sharpness of the Moon's outline,
and an impression that the dark Moon was very close to us.

Graphic Exhibitions of Unsteadiness.

If it were necessary to add graphic demonstration to the
effect of the disturbed atmosphere already described, we
have it in the photographs referred to in the following note
by Warren de la Rue,1 where he describes the method of
photographing the Sun's disc, in a very small fraction of
a second. He says, "So rapid is the delineation of the
Sun's image that fragments of the limb, optically detached
by the ' boil ' of our atmosphere, are frequently depicted
on the collodion, completely separated from the remainder
of the Sun's disc; more frequently still from the same
cause the contour of the Sun presents an undulating line."
To this case can be added a photograph of the Sun taken at
the Lick Observatory January 14, 1897; wherein the facti-
tious limb is like a nocculent border of cotton or wool with
ragged holes through the relatively faint, irregular outline

1 Proc. Brit. Assn. Adv't. Sci., Aberdeen, 1859, page 151.

M.-P.— Vol. L] DAVIDSON— APPARENT PROJECTION, ETC. 93

of the disc; and several cloud-like areas actually detached.
The breadth of the thin border is about 6", and the whole
factitious breadth must be more.

It seems a physical impossibility that we should be able
to detect any irregularities of even a dense atmosphere at
the border of the Sun. If that body were to decrease 45
miles in diameter, our instrumentation would not be able to
measure it under present conditions. Much less could we
detect it if there were actually rolling billows of the exterior
matter of the Sun 45 miles high around its border. They
would subtend but 0.1" in height; a quantity covered by the
finest spider thread in the telescope. Much less could we
detect any changes of atmospheric conditions at the dis-
tances of Jupiter, Mars, Venus, or Mercury. Some great
deep Sun spot just on the edge of the Sun would doubtless
show a depression, but not any imaginable storm disturbance
of the densest atmosphere, as we understand storms and
waves.

Official Recognition of the Factitious Borders of
the Moon and Sun.

In still further confirmation of the existence of the
spurious limbs of the Moon and Sun under unfavorable
atmospheric conditions, it is only necessary to appeal to the
representative of astronomical authority in the American
Ephemeris or Nautical Almanac. In that government pub-
lication an empirical correction of 2". 5 is applied to the
Moon's semi-diameter in order to represent the observed
value in meridian instruments. But this " constant is
omitted in the computation of solar eclipses and occultations
as due to telescopic and ocular irradiation" (p. 528,
1899). This 2". 5 of increased semi-diameter may represent
the average measure of the factitious border, but certainly
it is frequently much greater.

In many years of experience upon the Pacific Coast of
the United States in observing lunar transits, we have
learned to expect wild results when a markedly factitious
bright limb of the Moon was observed, the resulting longi-
tude being always affected as if the diameter of the Moon

94 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

were too great. At times, when the vibration or undulation
of the disc of the Moon was not rapid, the sharp, solid
limb could be observed even with a faint, spurious, vibrat-
ing outline beyond it. In i889-'90, during lunar transit
observations for longitude at San Francisco, our party had a
case of spurious disc from excessive diffusiveness, with a pre-
dicted error in the resulting longitude that was verified
upon reduction. This clearly indicated that even the offi-
cial empirical correction may frequently be too small;
while in times of supreme steadiness of the atmosphere and
resulting sharpness of the limb of the Moon, this correction
is not required.

If this factitious diameter belongs to the Moon, the Sun
is much more likely to carry a more pronounced factitious
diameter, and every observer must have frequently watched
its wildlv boiling limb flaring and leaping across the threads
or rolling forward like waves of flame along the horizontal
thread. This factitious border is in effect tacitly acknowl-
edged in the American Ephemeris above referred to, where
"the adopted semi-diameter of the Sun at the Earth's mean
distance is 16' 02". In the computations pertaining to
eclipses, Bessel's semi-diameter 15' 59". 788 has been
adopted." It appears to us that the explanation in the
American Ephemeris of the cause of the acknowledged
spurious diameter of the Moon is erroneous.

Great Elevations for Observation.

Although our experience has led us to advocate the selec-
tion of great elevations for astronomical observations of
precision and research, yet elevation alone is not a panacea.
The locality must present other favorable orographical con-
ditions. Unfavorable conditions are great gulches, canons,
or narrow valleys lying directly under a summit and leading
to it. These gulches become filled with highly heated air
during two or three days of clear, calm weather, and the
first wind from below drives this heated air over the summit
and spreads stars out into unsteady nebulous films; the
the rings of Saturn become woolly girdles, the crape ring is

M.-P— Vol. I.] DA VIDSON— APPARENT PROJECTION, ETC. 95

invisible and the details of the belts of Jupiter are indistin-
guishable. And in such a location during daytime, obser-
vations of precision on the Sun are absolutely useless.
The seeing upon a mountain peak may be generally good
when there is no snow on the surface; and even remarkably
good with snow, when the temperature is low and the wind
is strong; but when the Sun shines and the temperature
rises, and rapid evaporation takes place with light airs, the
seeing becomes very bad. (Experience at 10,450 to 7,250
feet elevation.)

On Mt. Conness, in the Sierra Nevada (12,566 feet ele-
vation), the geodetic heliotrope images were perfect until
the afternoon Sun shone upon the 2,300 feet nearly vertical
western face of that curious buttress; and then the uprising,
irregularly warmed atmosphere caused the images to become
unsteady. During the quieter moods of the atmosphere,
we observed Polaris for azimuth in the middle of the day
very satisfactorily with a telescope that would not show it
in daytime at lower elevations or under less favorable
conditions.

In 1872 we experimented in the Sierra Nevada at ele-
vations from 7,200 to 9,500 feet with remarkable success;
while another party of the United States Coast and Geo-
detic Survey sent for the same purpose to Sherman, 8,300
feet elevation, in the Rocky Mountains, had at times a very
unsteady atmosphere. The Lick Observatory at Mount
Hamilton, in the Diablo Range, at 4,209 feet elevation, is
surrounded by unfavorable orographical conditions, espe-
cially for observations of precision in daytime.

It would appear that Newton attributed the apparent
unsteadiness of celestial objects to the disturbance of our
own atmosphere. " Telescopes," he writes, "cannot be
formed so as to take away that confusion of rays which
arises from the tremor of the atmosphere. The only
remedy is a most serene and quiet air. Such as may per-
haps be found on the tops of the highest mountains above
the grosser clouds."

(3) November 21, 1900.

96 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

Precision Observations at Low Elevations.

And yet there are climatic conditions at comparatively
low elevations, which are supremely favorable for observa-
tions of precision at night. The experience of the observ-
ers of the United States Coast and Geodetic Survey on
the immense, elevated, arid plains from El Paso to San
Diego is very instructive. During the hot cloudless days
the atmosphere over the immediate surface of the parched
earth was in violent unsteadiness, and the heliotrope images
at even short distances flared out wildly like burning
houses. They were the worst possible objects for observa-
tions of precision. After sunset the conditions were sud-
denly changed. With a cloudless sky and a minimum of
aqueous vapor in the attenuated air (Barometer about 26
inches) radiation was quickly effective. The temperature
fell many degrees in a short time, and the air became
supremely quiet. The latitude telescope showed stars as
minute discs with diffraction rings running along the
micrometer thread with such extreme steadiness that it was
impossible to be in doubt more than one-tenth of a second
of arc in measurement; and in the transit instrument the
star marched with absolute regularity across the reticule.
It was an experience that an observer of over thirty years
in the field declared he had never before enjoyed.

Per contra, on the low Yolo plains of the Sacramento
Valley in California, we have had the azimuth signal-light
(distance eleven miles), in a calm night, running wildly up
and down the vertical thread of the theodolite through five
minutes of arc, in an ever changing line of broken stars of
the prismatic colors, and yet showing little or no horizontal
motion. The range of height of this column of images was
eighty-seven feet.

In the great Gangetic plains of India the night signals of
the triangulation parties, when shown from towers of fifty
feet elevation, frequently appeared as continuous columns
of light sixty feet high.

In the triangulation of the western coast of Lower
California by the United States Steamer "Thetis," the

M.-P — Vol. I.] DA VIDSON- APPARENT PROJECTION, ETC. 97

observer informs us that the unsteadiness of the atmosphere
and the irregularity of the refraction apparently lengthened
the signal poles to a hundred feet and more ; and the tripod
supports were like great writhing snakes. Even upon some
small low island in the ocean, where the climatic conditions
are favorable, the atmosphere at night becomes very quiet
and serene. From a vessel's deck we have watched the
larger stars nearly reach the horizon with very little
twinkling.

Recapitulation of Phenomena at Observation.

These examples are a few of the many experiences in
our geodetic and astronomical observations, and in the
descriptive experiences of a few other observers. And yet
from recent publications there appear to be many observers
who do not understand the cause of the phenomena, and
who seem to think there must be something occult and
unexplainable. We have therefore felt constrained to
repeat this explanation which we have held and announced
for many years. It seems to cover every phase of the
phenomena.

In the occultation of stars by the Moon, a spurious and
factitious limb of the Moon can be formed only and solely
by the unsteadiness of our atmosphere more immediately
surrounding the station of the observer.

In a serene atmosphere, the outline of the Moon is so
sharp and clear-cut that the mountains and valleys thereon
are very distinctively exhibited, and will bear the largest
magnifying power. When the atmosphere begins to change
to unsteadiness, the sharp outline of the Moon (or other
object) is first affected by tremors or shiverings, which are
so infrequent that the observer is able to select the actual
border and its features. As the unsteadiness of the atmos-
phere increases, the tremors increase in frequency and in
amplitude, until the border of the Moon becomes a con-
fused, blurred outline, in which no serrations can be
detected. The early shiverings usually denote the direction
of movement of the disturbed air. Under the atmospheric

98 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

conditions of clearness and serenity, the apparently ap-
proaching image of the star is a beautiful, steady, minute
disc with one or more delicate diffraction rings. The
Moon's white and sharply defined border exhibits all its
serrations and irregularities. As it approaches the star,
the diffraction rings disappear in the increasing light in the
field of view, but the disc remains; and at their visible con-
tact the disappearance of even the largest and brightest star
is absolutely instantaneous; and no observer can be in
doubt of the epoch beyond the tenth of a second. Under
such favorable atmospheric conditions, the image of the star
never enters upon the visible limb of the Moon, no matter
what the size of the objective or the magnifying power
employed. And with such conditions, the reappearance of
a star from the bright limb of the Moon will be instan-
taneous. If the star were apparently to make a near
approach to the Moon along the dark north or south limb,
where there are serrations, it might pass so close as to dis-
appear behind the first mountain and reappear in the next
valley (each phenomenon being absolutely instantaneous) to
be swallowed up by the next mountain, or in its absence, to
continue its visible course. We have heard (1846) the
elder Bond describe such a phenomenon in his experience
and it has happened once to ourselves. But when the
atmosphere begins to change to unsteadiness, the star loses
its diffraction rings, the nucleus is broken up and gradu-
ally diffused into a nebulous image, fuzzy and unsteady,
with a brighter, irregular, dancing nucleus, or is spread out
as a nebulous film as much as forty or fifty seconds of arc
in diameter. This unsteadiness of the atmosphere throws
the disc of the Moon into irregular vibrations or displace-
ments of equal amplitude and duration, giving it a factitious
border. With this disturbed and spurious limb the Moon
approaches the diffused image of the star, both being in a
state of great unsteadiness ; but if the nucleus of the star
be sufficiently large, bright, and colored, like Antares and
Aldebaran, the impression of its image upon the retina of
the eye is naturally more intense as an isolated spot than

M.-P.— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. 99

the image of the whitish, confused, spurious border of the
Moon, even when the former is within the latter. The
action of the eye is unconsciously selective, but a certain
effort of selection is made under the less favorable condi-
tion when the star is small or white. There is no effort at
selection needed to receive an impression of the Moon's
limb, although the actual limb may doubtless be detected,
under high powers, inside this factitious border. The im-
pression of the image of the star is, therefore, continuous
within the range of amplitude of the excursions of the disc
of the Moon, but is instantly lost when the limit of this
range of vibration is actually reached by the apparent
nucleus of the equally unsteady star, and the true limb of
the Moon occults it. With a large colored star all the
phases of this phenomenon are unmistakable; with a large
white star they may be somewhat in doubt, as in the case
of 47 Librae and other stars elsewhere quoted; with a small
white star they will probably not be noted, especially in
small telescopes with low power.

The exhibition of the phenomenon at the reappearance of
a star on the blurred and factitious bright limb of the Moon
should be in exactly the reverse order of the foregoing phe-
nomenon of disappearance; and we have been fortunate in
looking over our old record to find the original notes of the
disappearance of Aldebaran behind the dark limb of the
Moon, and its reappearance from the bright limb, on March
29, 1887, as elsewhere mentioned.

In this resume we have confined ourselves to the exhibi-
tion of the normal, and abnormal conditions in our atmos-
phere attending the phenomenon of occultations of stars by
the bright limb of the Moon, or by Jupiter or any other
planet, because the explanation virtually covers all associ-
ated phenomena in solar eclipses, lunar transits, transits of
Venus and Mercury, and the vagaries of star images.

When our atmosphere is absolutely serene there is not
and cannot be the slightest abnormal exhibition of form,
size, march, or steadiness of images; or of doubt of instanta-
neity in the epochs observed. Under such a condition there

IOO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser.

would be no empirical corrections in the Nautical Almanac
to the semidiameters of the Sun and Moon; and the diame-
ters of the planets measured by different observers, and
the right ascensions and delineations of the stars would be
consistent within the instrumental and personal equations.

Other Important Results Affected.

There is another and very important class of astronomical
results affected by these factitious borders of the Sun and
planets and the unsteadiness of the stars.

Dr. Newcomb says that the observed right ascensions of
the mean of the Sun's two limbs, relative to the fixed stars,
are affected by personal errors for which no means of elim-
ination have been tried. He suggests personal error and
possibly the effect of the Sun on the telescope. To these
causes should be added the larger effects of the disturbed
and factitious limbs; and in cases of steadiness, to diffrac-
tion of the limb at the spider line. He elsewhere notes
"very large errors, both accidental and systematic, to which
observations of the Sun are liable;" and again he regards
"the constant error in declination of the Sun as something
peculiar to the observer and the instrument." In another
place he says: "There is a remarkable systematic differ-
ence in the observed A. R. of Mercury, according as the
planet is east or west of the Sun, and therefore according
to the illuminated side. The sign of the result shows that
the reduction to the center of the planet was apparently too
small, and then it is of interest to learn according to what
law this error changed as the planet moved around its rela-
tive orbit."
^ Furthermore, the hurtful effect of observing upon the

^Jt-fJU- or Ls*i---t*L**-%m^ outer Ht»b of the factitious limb of the disturbed Moon can

be shown in the determination of the parallactic inequality
of the Moon from meridian observations. Dr. Newcomb
says the method is peculiarly liable to systematic error,
owing to the fact that observations have to be made on one
limb of the Moon when the inequality is positive, and on
the other limb when it is negative. Hence if we determine

M.-P— Vol. I.] DAVIDSON— APPARENT PROJECTION, ETC. IOI

the inequality (125". 5) by the comparison of its extreme
observed effects on the Moon's right ascension, any error in
the adopted semidiameter will affect the result by its full
amount. This suggests that this gravitational method of
determining the Solar Parallax would be more accurately
employed by observing occultations of stars large enough
to be seen through the spurious bright limb of the Moon.
Moreover, the observations themselves would afford some
data for the elimination of the error depending upon a fac-
titious limb.

Nearly all Measured Diameters Too Large.

In all instrumental observations for the right ascension of
the Sun or Moon, and for their declination and diameter,
it must be evident that the observer obtains accurate meas-
ures only when the disc of either body is sharply defined,
devoid of tremor or unsteadiness, and unaffected by irregu-
lar and extraordinary refraction, without reckoning diffrac-
tion at the spider line, unknown instrumental errors, and
peculiarities of observation. As these supreme conditions
of steadiness are seldom obtained, it necessarily follows that
the mean of any number of observations taken under differ-
ent atmospheric conditions (say for example the diameter
of the Sun, Moon, or planets) must be too large. The
diameter can only be too small through error of observation,
instrumental errors, diffraction of the spider line, or abnor-
mal refraction.

With a disturbed atmosphere in observations for the deter-
mination of the right ascension of the Moon during the first
half of a lunation, the observed A. R. of the limb will be
too small and for the second half too large. Similar results
will follow from observations of the I and II limbs of the
Sun and planets. Therefore all published diameters of the
Sun, Moon and planets derived directly from actual obser-
vations must be too large. Moreover, this presents a case
where the mean of measured quantities is not the most
probable value. The mean of all the minimum measures
would be nearer the truth.

102 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Sbr.

The amplitude of the excursions of the images of the
Sun, Moon, and planets in a disturbed atmosphere is a vari-
able quantity that can not be determined but should be
avoided if practicable. To obtain the most trustworthy
results in reasonable time, we need the fixed observatories
located at points which conspire to give the best atmospheric
conditions determined upon after systematic and exhaustive
trial for special lines of research ; the highest class of instru-
mentation; observers with ideal eyes (of which unfortu-
nately there are very few) ; discrimination in the selection
of proper times of observation; and a wise rejection of
observations made under abnormal conditions.

Note. Since this paper was presented to the California Academy of
Sciences, Professor A. E. Douglass of the Lowell Observatory at Flagstaff,
Arizona, has published the second of his interesting papers upon "The
Atmosphere, Telescope and Observer," and "Scales of Seeing." Pop.
Astr. 1898.

MATHEMATICAL-PHYSICAL.
Vol. I.

No. i— On Rational Quadratic Transformations. By M. W. ^j
Haskell j

No. 2— The Quadratic Cremona Transformation. By Leonard E. f $.35
Dickson !

No. 3 — On Curvilinear Asymptotes. By M. W. Haskell J

No. 4— Systems of Simple Groups derived from the Orthogonal

Group. By Leonard E. Dickson .25

No. 5 — Systems of Simple Groups derived from the Orthogonal "^
Group. (Second Paper.) By Leonard E. Dickson !

No. 6 — On an m n2 Parameter Group of Linear Substitutions in {
m n Variables. By E. J. Wilczynski J

No. 7 — The Apparent Projection of Stars upon the Bright Limb^J
of the Moon at Occultation, and Similar Phenomena at (
Total Solar Eclipses, Transits of Venus and Mercury, {
Etc., Etc. By George Davidson, Ph.D., Sc.D J

All subscriptions, applications for exchanges, and inquiries concerning the
publications should be addressed to

J. O'B. GUNN, Corresponding Secretary,

California Academy of Sciences,

San Francisco, California.