LAURENS: MONOCHROMATIC LIGHTS. 259 



A. The Light-Generator. 



The light-generator may be described as follows: the sources of 

 light were Nernst glowers on a 220-volt circuit, the light from which 

 passed, first through a rectangular opening in a diaphragm of black- 

 ened sheet iron (C), then through a converging lens (D), which was 

 at such a distance from the source of light that the conjugate foci were 

 at equal distances from the lens, and finally through a large glass prism 

 (F) filled with carbon bisulphide, placed within the focal distance of 

 the lens, and at the angle of minimum deviation. The spectrum thus 

 obtained was cut down by diaphragms of blackened cardboard with 

 narrow vertical slits of appropriate size and position. These dia- 

 phragms were placed in a holder (G) at the focal points of the several 

 lights used. Side reflections were eliminated by enclosing light, lens, 

 prism, etc., in a covered box (//, /, J, K), which was blackened inside, 

 and completely closed except at the end farther from the source of 

 light, where the light from the prism was projected into the dark 

 chamber. Every care was taken, by the use of suitable screens, 

 etc., to exclude from the dark chamber all light except that proceeding 

 from the prism. It was possible to revolve the generator on a pivot 

 at P, so that the direction of the light could be changed within certain 

 limits. An adjustment at A' enabled the experimenter to change 

 slightly sideways the position of the lamp. The colored lights used in 

 the experiments were four in number, as follows: blue, 420-480 fx/j.; 

 green, 490-550 /jl/j.; yellow, 570-620 yu^u; and red, 630-655 /^/x. The 

 terms blue, green, yellow, and red are used for convenience in desig- 

 nating these lights, and not in their strict physical application. 



B. The Combined Apparatus. 



The complete apparatus, as shown in Figure 1 , consisted of two such 

 light-generators, placed at opposite sides of the dark chamber (L), 

 into which the light from each was projected, passing first through a 

 plate-glass window (M) in a screen (N), the window (M) being of 

 similar size and thickness to that in the box which surrounded the 

 radiomicrometer by which the light was originally measured. The 

 walls and ceiling of the dark chamber were of opaque black cloth, 

 there being an aperture (0) through which the observations were made. 

 The height of this dark chamber was 70 cm., and its floor area 130 X 



