Chromatic Curves of Microscope Objectives. By E. M. Nelson. 9 
monochromatic light apparatus * I am able to fill the back lens of 
the apochromatic condenser with any light I please of an approxi- 
mately uniform wave-length, and then by placing a delicate test 
object on the stage, and with a deep eye-piece, focal differences of rays 
of various wave-lengths can be directly determined ; the size of the 
illuminating cone can be altered at pleasure, and beams of various 
obliquities used. 
There are two methods of measuring the focal differences, (a) by 
means of the graduations of the fine-adjustment, (b) by receiving the 
image on a screen, and noting the differences in screen distance. 
When one focal difference is known, the other may be calculated. 
Thus let d be the difference of focus at the object side, or the move- 
ment by the fine-adjustment, and let D be the difference of focus at 
the image side, or difference of screen distance, and m the initial 
power of the objective, w being the screen distance, and assuming 
that w is large compared with D. Then t 
T>W d w 1 , 2 
D = = dm 2 
p 
and d = 
D/ 2 D 
w* 
I have measured the foci for the lines B, D, E, F, and G- of many 
various objectives both ancient and modern, and now submit for 
your inspection the curves of some of them which possess particular 
interest, drawn from those measurements (fig. 1). 
But before discussing these curves let me point out some 
important points which bear on practical microscopy in connection 
with monochromatic illumination. 
The ends in view with illumination by monochromatic light have 
been, speaking for myself — and I think it is also the generally received 
impression — ( a ) the increase of resolving power by means of illumina- 
tion of the object by light of a shorter wave-length than usually 
employed, and ( b ) the removal of the secondary spectrum in achromatic 
lenses. But there has been a certain mystery respecting the effect of 
monochromatic illumination, the solution of which has not been clear. 
As it is a most important subject, 1 hope you will pardon me if it is 
dealt with at some length. 
As stated previously,! the effect of shortening the wave-length 
was practically to add *1 N.A. to the aperture* of the objective 
mentioned. For if A, is the number of waves of light per inch, and 
N.A. the numerical aperture of the objective, then L the number of 
lines resolved per inch will be equal to 2 A (N.A.). As vision fails 
greatly in blue light, and glass is not transparent for very short 
waves, anything as high up as the Gr line is quite out of the question, 
so we must content ourselves with a wave-length lower down the 
spectrum. In practice, shortening the wave-length gives about 
* Journal R.M.S., 1892, pi. I. f Tom. cit., p. 341. 
X Tom. cit., p. 44G. 
