[ 547 ] 



XVII. Colour Photometry. Part II. The Measurement of Reflected Colours. 

 By Captain ABNEY, C.B., R.E., F.R.S., and Major-General TESTING, R.E., F.R.S. 



Received May 3, Read May 31, 1888. 



[PLATES 20-23.] 



XXVII. Old Method of Measurement* 



IN our first paper on this subject we have shown how the luminosity of the spectra of 

 various sources of light can be measured ; and the present paper is an extension of the 

 subject, dealing with the measurement of the light reflected from bodies in terms of the 

 colours of the spectrum of the light illuminating them. By the method which we 

 adopted in the first part of " Colour Photometry " this can be effected, and, indeed, we 

 carried that out in several instances. The method then employed was very simple. If 

 we wished to measure the illuminating value of the spectrum of light reflected from a 

 metal, we placed it at an angle in front of the slit of the spectroscope, so as to reflect 

 the light from the crater of the positive pole of the electric light through the photometer, 

 and measured the luminosity of each part of the spectrum thus formed by the method 

 we indicated in our paper. Again, in experimenting with GORHAM'S discs, such as 

 MAXWELL employed, where it became necessary to determine the light reflected from 

 the different coloured papers or cards used in the discs, the plan first adopted was to 

 replace the receiving shadow screen of zinc oxide (see VI) by the coloured papers, 

 and again to make a luminosity measurement. This plan answered its purpose, but it 

 was rather laborious. When two or three colours are combined by rotation to form a 

 grey, and black and white sectors are combined to match that grey, in order to 

 ascertain the total luminosity of each colour, the angular value of the sectors being 

 known, it is necessary to refer the luminosity to that of some standard reflecting 

 surface, which is naturally a white one. As the comparison light is coloured by 

 falling on coloured paper, the value of the spectrum reflected from such paper could 

 not by this first method be directly compared with that reflected from the white 

 screen. In the case of a coloured screen, the curve of spectrum luminosity would 

 therefore have to be reduced to that in which the comparison light was white. This 

 difficulty was surmounted by making half the receiving screen white and half of the 



* Tho numbering of the paragraphs and figures in this paper is a continuation of that of Part I. 

 Bnkcrian Lecture. ' Phil. Trans.,' 1886. 



4 A 2 22.12.88 



