ON COLOUR PHOTOMKTRY. 567 



The intensities were obtained by substituting for the first small glass prism in front 

 of the slit apiece of the metal whose reflection was to be examined. The two images 

 were received on a white screen and equality established, as before described. A very 

 interesting confirmation of the accuracy of measuring the illuminating value of 

 different light was found in the case of burnished copper. In 1886 the " luminosity 

 curve " of the spectrum of the light reflected from copper was obtained, the piece of 

 copper used being part of the same plate from which the intensity curve was obtained 

 in our recent experiments. The dots close to the curve of luminosity derived from 

 the intensity curves show the agreement of the results obtained by two very different 

 methods. In the same year the luminosity curve of emerald green was made by our 

 former plan, and on reducing the curve in the proper proportion, so that the maximum 

 coincided with the maximum of the luminosity curve obtained from the comparison of 

 the intensity of light reflected from the emerald green with that from a white surface, 

 it was found that the agreement was extremely close. These coincidences confirm the 

 y accuracy and value of our method of measuring the luminosity of light of different 

 colours. 



XLII. Comparison of Reflection from, unth Transmission through, a Pigment. < 



We wished also to ascertain whether the light reflected from a pigment was 

 identical with that transmitted through the same. In some cases, when there is 

 quasi-metallic reflection, in parts this cannot be the case ; but in ordinary colours this 

 reflection is not present. We tried several, and came to the conclusion that the 

 transmitted and reflected lights are of the same character. Fig. 28 gives the result of 

 one such experiment made with Prussian blue. The transmitted light was measured 

 by placing in the lower spectrum a thin film of gelatine impregnated with the pigment. 

 The curves of the figure were constructed from the following table. 



