1911 - 12 .] Absorption of Light by Inorganic Salts. No. V. 41 
on the photographic plate. It has been superseded by the apparatus to 
be described in this paper. 
With a glass prism or rock-salt prism minimum deviation is not so 
important as with quartz, because they do not doubly refract. With a 
Cornu prism we get minimum deviation only at one point on a photo- 
graphic plate ; also, when using a thermopile, minimum deviation is not so 
necessary, since the radiation receiving surface is not very narrow. I have 
therefore let fall the condition that there should be automatic minimum 
deviation, and have devised the simple universal apparatus represented 
here, which is suitable for radiation measurements throughout the infra- 
red, visible spectrum, and ultra-violet, and incidentally as suitable for 
photography in the ultra-violet as the instruments on the market at 
present. It is with this apparatus that the observations recorded in this 
paper were obtained. 
The sketch represents only the essentials. The slit S is attached to a 
brass tube sliding in a wooden block and clamped in position by a screw. 
The thermopile and the photographic plate carrier with the two brass 
grooves for holding the slide are shown at the side. They slide between 
the guides shown, the screw passing through the slot. They are fixed 
in position by tightening a nut on the screw. The light from the slit is 
rendered parallel by the concave mirror M, passes through the fixed prism, 
