ORTHOCHROMATIC’ PHOTOGRAPHY. ZS 
The wave-lengths in the above table are those given by 
Rowland, and are usually employed, but for extreme accu- 
racy in spectroscopic work corrections must be used. The 
unit of measurement is Angstrom’s unit, one ten millionth of 
a millimeter or A. U. (called also tenth meter, t. m.), and 
only round numbers are given; thus, taking the red cadmium 
line, it would be called on the above plan 6438, while its 
correct measurement is 6438.4722. It is frequently the 
custom to employ other units, for instance, the millionth of 
a millimeter mp is used, or the thousandth of a millimeter p 
(pronounced mu). Thus the cadmium line may be expressed 
as: 6.4384722 x 10-° cm, or 0.64384722 p, or 643.84722 py 
or mp, or 6438.4722 A. U. or t. m. It is usual in photo- 
graphic literature to use the abbreviated A. U., as 6438 or 
the still briefer designation 644. The Greek lambda or A is 
used to designate the wave-length. 
It is very frequent also to designate a colour or position in 
the spectrum by such a term as D ¥% C, which means half way 
between the lines D and C; this would mean, taking 5893 as 
the mean wave-length of the D lines, that the stated position 
would be half the distance or difference of wave-lengths, thus 
C 6563 — 5893 = 670, then 670 divided by 2 plus 5893 = 
6228, the designated wave-length. 
DISTRIBUTION OF COLOURS IN THE SPECTRUM.—Various 
tables of the distribution of the colours in the spectrum have 
been given, but the following compiled by Listing may be 
accepted as being typical: 
Colour Wave-length 
End 819 
Deep red Middle 768 
End 723 
Red Middle 683 
End 647 
