MONOCHROMATIC LIGHT OF ANY DESIRED WAVE-LENGTH. 
915 
of dispersion by the other portions of the spectrum in the field of view. In cases 
where the dispersion of the axes is great the method fails altogether, for the inter¬ 
ference figures become perfectly unintelligible. 
From the above discussion of the methods hitherto adopted, it will be apparent 
that the ideal arrangement must be one by means of which the ivhole Jield of the 
optical instrument is evenly illuminated with light of as nearly as practicable one 
wave-length, which may be rapidly varied, as desired, from one extreme of the 
spectrum to the other. The apparatus now described enables these conditions to be 
fulfilled. It was suggested by the arrangement described by Abney,"^ and employed 
in his researches, in conjunction with Festixg,! upon colour photometry. 
Abney’s arrangement consists essentially of a spectroscope with two prisms, but 
with the eye-piece of the observing telescope replaced by a screen, upon which the 
spectrum is received, and which is perforated by a movable and adjustable slit, 
through which any desired portion of the spectrum may be allowed to escape. This 
slit of monochromatic light is allowed to fall upon a lens of comparatively large dia¬ 
meter, and. of such convenient focal length that an image of the nearest surface of the 
second prism may be thrown upon the screen which it is desired to illuminate, in the 
form of a uniform patch of light involving fewer wave-lengths the narrower the slit. 
The position of the screen with relation to that of the lens is such that the successive 
patches of colour all illuminate the same space upon the screen. 
The arrangement now described, while similar in principle to that of Abney, differs 
from it in certain important particulars rendered necessary by the exigencies of crys- 
tallographical optical work. The chief differences and innovations are as follows :—• 
1. Instead of desiring to illuminate an opaque screen, to be observed by reflection, 
it is desired to employ the beam of monochromatic light in directly illuminating the 
field of an optical instrument, the polariscope of an axial angle goniometer for 
instance. Hence the large lens, so conveniently used by Abney to direct the coloured 
light upon a screen, is discarded, and the objective of the observing instrument is 
brous’ht to within an inch or so of the exit slit, thus utilising' the whole of the issuing 
coloured light and economising space, 
2. Instead of a movable slit, which, the lens being discarded, would necessifate a 
corresponding but highly inconvenient movement of the observing instrument in order 
that the issuing light for all the different colours should always pass along its ojjfical 
axis, the exit slit is fixed. 
3. The different colours of the spectrum are caused to pass the fixed exit in succes¬ 
sion by rotation of the dispersing apparatus. This latter consists, instead of two 
prisms as employed by Abney, of one large 60° prism whose faces are capable of 
receiYing almost the whole of the light from the collimating lens of two inches 
* ‘Phil. Mag.,’ 1885, vol. 20, p. 172. 
t ‘ Phil. Trans,,’ vol. 177, p. 423. 
6 A 2 
