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XVIIT. Colour Phenomena in Photometry . 

 By J. S. Dow, A. C. G.I., B.Sc.* 



THE discussion o£ Dr. Fleming's paper on Photometry, 

 read before the Institution of Electrical Engineers in 

 1903, revealed great differences of opinion on the importance 

 of colour phenomena in photometry, and it still does not 

 seem to be generally known to what extent they are noticeable 

 under ordinary working conditions. This may be due to the 

 fact that most of the work done on this subject was carried 

 out with special apparatus, such as the spectrophotometer, 

 and not with the ordinary implements of photometry. 



It therefore occurred to the author that some simple ex- 

 periments on these points, carried out by him at the Central 

 Technical College on an ordinary photometrical bench, might 

 be of interest. 



The sources of light were two similar glow-lamps, which 

 could be screened with glass of different colours, and which 

 were compared by means of one or other of several different 

 photometers in the usual way. Four photometers were made 

 use of during the experiments — the Lummer-Brodhun, the 

 Grease-Spot, the Joly, and the Flicker. 



'The uncertainties which may be introduced by colour 

 phenomena appear to be due to four separate effects : — 



(1) The difficulty experienced in forming a judgment in 

 the case of differently coloured lights, and the possibility 

 that the judgments of different people may not be the same. 



(2) The fact that the apparent relative brightness of two 

 surfaces, illuminated by light of different colour, depends 

 on the part of the retina on which the image of them is 

 received. 



(3) The Purkinje phenomena. 



(4) The possibility, when mirrors are made use of, that 

 the coefficient of reflexion may not be the same for different 

 coloured lights. 



(1) No doubt people differ in their capacity in this respect, 

 but according to the author's experience it is chiefly a matter 

 of practice. After a considerable amount of practice, he has 

 found that it is possible to secure fairly consistent results 

 even when comparing such oolours as ruby-red and signal- 

 green, while anyone unused to such work would be quite 

 unable to do so. Extraordinarily consistent results were 

 sometimes obtained, but it was found that if the observer 

 stopped work for an hour or so, his readings afterwards 



* Communicated by the Physical Society : reed May 25, 1906. 



