6 DOCUMENTS, AND THEIR SCIENTIFIC EXAMINATION.

WATKINS' TABLE OF RELATIVE EXPOSURES FOR
PHOTOGRAPHIC DOCUMENTS.

A. Reduced Scale.

Number of focal lengths between lens and
a
2.2
2.4
2.6 1
2.8
3.0
3.5
4
5
6

document.

Relative exposure com-

pared with that with lens at normal ex-
4
3.26
2.93
2.64
2.42
2.25
2
1.76
1.56
1.44

tension.

B. Enlarged Scale.

Number of focal

lengths between
2
2.2
2.4
2.6
2.8
3,0
3.5
4
4.5
5
5.5

lens and plate.

Relative exposure

compared with that with lens at
4
4.84
5.76
6.76
7.84
9
12.25
16
20.25
2.5
29.2

normal extension.

1

Colour-Sensitive Plates and "Screens.—The use of
colour screens with colour-sensitive plates is essential
for ^he photography of the details of certain documents,
especially in the case of writing in blue or violet pencil
pigments, or when the paper itself is of a yellow tint,
For example, the writing in copying-ink pencil pigment
on the wooden stave (p. 136) required the use of a yellow
screen and an appropriate plate to obtain a satisfactory
result. This method is also useful in certain cases for
demonstrating differences between certain kinds of ink
which appear of the same tone, upon paper.