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, ete., in a covered box (/, I, 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 X 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 uy; 
green, 490-550 wu; yellow, 570-620 wu; and red, 630-655 uu. 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 (LZ), 
into which the light from each was projected, passing first through a 
plate-glass window (JM) in a screen (JV), 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 (QO) through which the observations were made. 
The height of this dark chamber was 70 cm., and its floor area 130 X 
