1892.] Colour Photometry. 369 



II. "Colour Photometry. Part III." By Captain W. DE W. 

 ABNEY, C.B., R.E., D.C.L., F.R.S., and Major-General 

 FESTING, K.E., F.R.S. Received December 14, 1891. 



(Abstract.) 



The authors refer to their paper on Colour- Photometry (Bakerian 

 Lecture, 1886), in which a method was given of forming a curve of 

 luminosity of the spectrum, the source of light being the crater of the 

 positive pole of an electric arc lamp. 



They point out that in making the observations for forming this 

 curve no attention was paid to the part of the retina of the eye which 

 was used, and which embraced the " yellow spot " and some of the 

 surrounding portion. 



In their further researches this point came to be of importance, 

 and they describe how, by modifications of the apparatus and of the 

 methods of observing, they were able to use either the yellow spot or 

 portions outside it, and they give the results of the observations, 

 showing how the curves become modified in each case. 



The absorption by the yellow spot takes place in all rays more 

 refrangible than E ; but to the less refrangible rays the outer part of 

 the retina is less sensitive than the central part. 



The Limit of Colour Vision. It is well known that when light of 

 any colour becomes enfeebled to a certain degree, the eye fails to see 

 colour, though it may still recognise the existence of light. Observa- 

 tions were made to determine the point at which, for each part of the 

 spectrum, the sensation of colour is lost. The same apparatus as 

 before was used for forming the spectrum and the " reference " beam 

 of white light, and a supplementary apparatus was devised for reduc- 

 ing the beam of light and for measuring the amount of reduction. 

 Each coloured beam was reduced until, in comparison with a feeble 

 white beam, it appeared colourless. The amount of reduction in 

 each case being measured, a curve was plotted showing the propor- 

 tional reduction in part of the spectrum. The absolute intensity of 

 the beam from D having been measured by comparison with an amyl 

 acetate lamp, that of each other part of the spectrum was calculated 

 by aid of the luminosity curve above referred to. It then became 

 possible to plot a curve which shows the intensity of the original 

 source at which, in each part of the spectrum, colour first becomes 

 visible. 



The portion of the spectrum in which colour is visible from the 

 feeblest source is between about X 500 and A, 615. This accounts for 

 the fact that in a feeble light, such as that of the moon, objects appear 

 to be of a greenish hue, and also that moonlight passing through 

 coloured windows does not give a coloured image in most cases. 



