2 
Transactions of the Society. 
absorption medium,* which yields very approximately monochromatic 
yellow-green light (X 560). It is a very simple and agreeable 
medium for visual work, and at the same time it greatly improves 
the definition of ordinary achromatic objectives. It must be re- 
membered that as it only passes light low down the spectrum it will 
not increase the resolving power of an objective. The spectra are 
not quite so far in as those in fig. 2, and they are, moreover, tinged 
with blue on their inner, and with red on their outer edges.] f 
I have not given a figure to show the spectra under the conditions 
of ordinary lamp-light illumination because it would be very similar 
to fig. 4 ; the only difference being that the dioptric beam would be 
white, the red would be a little brighter, and the blue a little dimmer 
than in fig. 4. In ordinary illumination the visual image is com- 
posed of neither the red nor the blue, but is that of rays such as orange 
and green. The advantage of the interposition of blue absorption 
media is due to the fact that the image-forming rays are sent higher 
up the spectrum, and consist of yellow and bluish-green. Any one 
carefully inspecting these figures will be convinced of the decided 
advantage gained by the use of monochromatic blue light. It will be 
noticed that I have adhered to Abbe’s old nomenclature with regard 
to the spectra. 
Before closing I will give a few words on the management of the 
apparatus and refer to fig, 7. The bull’s-eye B will be found of 
great service as it requires great intensity of light to give a satis- 
factory illumination with the blue. The bull’s-eye B should be 
attached to the lamp L, and the two first pianos should be adjusted 
for parallel light before the third lens, the one remote from the lamp, 
is placed in position. In order to place the bull’s-eye and lamp in a 
proper position with regard to the slit in the screen S, a piece of 
white card should be held in front of the prism P, and the light 
adjusted on it. It will then be easily seen if the prism P is evenly 
illuminated by the strongest part of the beam, and also whether the 
lamp and the bull’s-eye are at a proper distance from the screen S. 
The prism P should be of dense flint, as crown does not give 
sufficient dispersion. Very great care must be taken to place the 
prism P at minimum deviation. This is best accomplished by causing 
the spectrum to fall on the screen D when the lens R is turned out 
of the way. The prism is now rotated on its axis, and it will be seen 
that the spectrum on the screen D moves so that the angle DPS 
becomes either more obtuse or more acute. When the angle D P S is 
greatest, then the prism is at minimum deviation. The point where 
the centre of the spectrum falls on the screen D should now be noted. 
The lens R should next be turned into such a position that the spec- 
trum may be in the same place as before. It is advisable to again 
move the prism, noticing particularly the slightest movements of the 
spectrum on the screen 1) to insure the deviation being at a minimum. 
* See this Journal, 1889, pp. 133, 700. t Added January 20th, 1892. 
