﻿122 



Mr. J. S. Dow on Colour 



even if the eye were kept at the same distance, the size of 

 the image would be different, and a new portion of the retina 

 would be covered by it. 



The position of the photometer, for which we obtain balance, 

 therefore depends upon : — 



(a) The obliquity at which rays from the illuminated 



surface strike the eye. 



(b) The distance away of the eye from the surface. 



(c) The size of the surfaces. 



The first point is not very important, for in focussing our 

 eyes on the surfaces we involuntarily look straight at them. 

 But (6) and (c) may easily affect the readings considerably. 

 This is brought out in the curves in fig. i. 



Pig. 1. — Ruby-Red Light compared with Signal-Green. 



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10 20 30 4-0 50 60 70 80 90 100 

 Distance of eye from field of Photometer in centimetres. 



(1) Lumnier-Brodhun Photometer (telescope removed). 



(2) Joly Photometer. 



(3) Joly Photometer (linear dimensions of blocks reduced by half). 



(4) Grease-Spot Photometer. 



These curves were obtained as follows : — Two glow-lamps, 

 screened with red and green glass respectively, were run at 

 a constant P.D., and compared by means of each of the three 

 photometers referred to above. 



