ON COLOUR PHOTOMETRY. 
455 
To effect this we made the light which had passed through the slit and the colli¬ 
mator to be our comparisondight by using the reflection from the first surface of the 
first prism. 
The general arrangement of the apparatus is seen in the accompanying diagram. 
Hid are rays coming from a heliostat, and a solar image is formed by a lens L l5 on 
the slit Sj of the collimator C. The parallel rays produced by the lens Lo are partially 
refracted and partially reflected. The former pass through the prisms PjPo, and are 
focussed to form a spectrum by a lens L 3 on D, a movable screen in which is a slit, So. 
The rays coming through So are collected by a lens L t to form a monochromatic image 
of the near surface of the second prism on F. When D is removed the image of the 
surface of the prism is white, obtained by tilting the lens L,. at an angle as shown. 
The reflected rays from P x fall on G a silver-on-glass mirror. They are collected 
by L 5 , and form a white image of the prism also at F. 
At M is a small electromotor carrying moving sectors as already described. 
It will be seen that both the comparison-light and the spectrum itself are formed 
by the light coming through the slit. Any small haze passing over the sun thus 
