Chromatic Curves of Microscope Objectives. By E. M. Nelson . 1 1 
sharpened up hy the use of monochromatic illumination. The P. 
angulatum in this respect forms an excellent example. 
Some lenses fail altogether with blue light because the optician’s 
aim has not been to correct the lens for the spherical aberration of 
the blue ray. The lens I am exhibiting to you this evening fails 
altogether with blue light. The apocliromatics do not go off like 
this in the blue, at the same time they do not yield the results that 
might be expected from them. We find therefore that the idea of 
the use of monochromatic light for shortening the wave-length, 
and by that means obtaining a greater resolving power, must not be 
pressed too far, especially when speaking of both apochromatic and 
achromatic lenses as they are at the present time. (In addition to 
this, account must be taken of the fact that the eye fails in the blue 
light, not to mention the pain caused by its prolongued use.) 
For visual purposes at least there are no expectations of any 
great advance with light of a wave-length much shorter than 1/50,000 
in., or the blue green. What may be done photographically with a 
lens, made of material which is transparent to rays high up in the 
violet, and whose spherical aberration for those rays has been specially 
corrected, when used with plates made sensitive to such rays, we 
cannot say ; but under existing conditions we must not look for a 
great percentage of gain (only 7 per cent, with a wave-length of 
1/50,000 in., and 14 per cent, in table with low and medium powers). 
This testing by means of monochromatic blue light shows directly 
the quality of an objective for photographic purposes. 
There are many lenses “ corrected for photomicrography ” which 
are all right so long as small cones are used, but directly any strain 
is placed upon the objective by a large cone the spherical aberration 
for the rays of short wave-length is instantly developed, It is here 
that a monochromatic yellow-green screen becomes of service because 
with specially prepared plates good photographs may be taken with 
almost any kind of lens.* Strictly speaking, the screen should be 
monochromatic for that special ray for which the optician has cor- 
rected the spherical aberration of the objective : this usually will be 
found to be the yellow green. I have been trying for some time 
past to find a monochromatic glass screen. With coloured gelatin 
films an excellent monochromatic screen may be obtained, but it is 
not so easy to match the gelatin in glass. I have however found 
two glasses t which answer the purpose very well, as you will be able 
to judge for yourselves by an examination of the image of the 
P. angulatum as shown with a cheap lens in the Microscope on the 
table. 
* It is of interest to note that the screen may be placed either between the lamp 
flame and the back lens of the condenser or over the eye-piece. It should be 
remembered that all Prof. Abbe’s diffraction experiments may he performed above 
the eye-piece, equally as well as at the hack of the objective. 
f Can be procured at Messrs. Baker’s. 
