[Nature, Vol. LXXXVIII. p. 377, 1912.]
IN his interesting address on spectroscopic methods, Prof. Michelson falls into a not uncommon error when he says that, in order to obtain a pure spectrum, " two important modifications must be made in Newton's arrangement. First, the light must be allowed to pass through a very narrow aperture, and, secondly, a sharp image of this aperture must be formed by a lens or mirror."
Both these modifications were made by Newton himself, and with a clear understanding of their advantages. In Opticks, Exper. 11, we read:—"In the Sun's Light let into my darkened Chamber -through a small round hole in my Window—shut, at about 10 or 12 feet from the Window, I placed a Lens, by which the Image of the hole might be distinctly cast upon a sheet of white Paper, placed at the distance of six, eight, ten, or twelve Feet from the Lens....For in this case the circular Images of the hole which comprise that Image...were terminated most distinctly without any Penumbra, and therefore extended into one another the least that they could, and by consequence the mixture of the Heterogeneous Rays was now the least of all."
And further on:—
" Yet instead of the circular hole F, 'tis better to substitute an oblong hole shaped like a long Parallelogram with its length Parallel to the Prism ABC. For if this hole be an Inch or two long, and but a tenth or twentieth part of an Inch broad or narrower: the Light of the Image pt will be as Simple as before or simpler [i.e. as compared with a correspondingly narrow circular hole], and the Image will become much broader, and therefore more fit to have Experiments tried in its Light than before."
Again, it was not Bunsen and Kirchhoff who first introduced the collimator into the spectroscope. Swan employed it in 1847, and fully described its use in Edin. Trans. Vol. xvi. p. 375,1849. See also Edin. Trans. Vol. xxi. p. 411, 1857 ; Pogg. Ann. C, p. 306, 1857.
These are very minor matters as compared with what Prof. Michelson has to tell of his own achievements and experiences, but it seems desirable that they should be set right. train passing at high speed at a distance of 'not more than 150 yardi almost inaudible. The wheels were in full view, but the situation was such that the line of -v passed for most of its length pretty close to the highly heated ground. It seemed clear tha sound rays which should have reached the observers were deflected upwards over their h which were left in a kind of shadow.kness, of which the first is similar to the final uniform medium, and the second similar to the initial uniform medium. •Of the three partial reflections the first and third are similar, but the secondo guide the lantern-plates into position, and thus to ensure their subsequent exact superposition by simple mechanical means.