1888.] 



Colour Photometry. 



237 



II. " Colour Photometry. Part II. The Measurement of Re- 

 flected Colours." By Capt. W. de W. Abney, R.E., F.R.S., 

 and Major-General Festixg, R.E., F.R.S. Received May 3, 

 1888. 



(Abstract.) 



In a previous paper we showed how the luminosity of different 

 spectrum colours might be measured, and in the present paper we 

 give a method of measuring the light of the spectrum reflected from 

 coloured bodies such as pigments in terms of the light of the 

 spectrum reflected from a white surface. To effect this the first 

 named of us devised a modification of our previous apparatus. 

 Nearly in contact with the collimating lens was placed a double image 

 prism of Iceland spar, by which means two spectra were thrown on 

 the focussing screen of the camera (which was arranged as described 

 in the Bakerian Lecture for 1886), each formed of the light which 

 enters the slit. The light was thus identical in both spectra. The 

 two spectra were separated by about §- of an inch when the adjust- 

 ments were complete. A slit cut in a card was passed through 

 this spectrum to isolate any particular portion which might be 

 required. The rays coming from the uppermost spectrum were 

 reflected by meaus of a small right-angled prism in a direction nearly 

 at right angles to the original direction on to another right-angled 

 prism. Both prisms were attached to the card. From this last prism 

 the rays fell on a lens and formed on a white screen an image of the 

 face of the spectroscope prism in monochromatic light. The ray of the 

 same wave-length as that reflected from the upper spectrum passed 

 through the lower half of the slit, and falling on another lens 

 formed another image of the face of the prism, superposed over 

 the first image. A rod placed in front of the screen thus cast two 

 shadows, one illuminated by monochromatic rays from the top 

 spectrum, and the other by those from the bottom spectrum. 

 The illumination of the two shadows was equalised by means 

 of rotating sectors which could be closed and opened at pleasure 

 during the time of rotation. The angle to which the sector required 

 to be opened to establish equality of illumination of the two shadows 

 gave the ratio of the brightness of the two spectra. When proper 

 adjustment had been made the relative brightness was the same 

 throughout the entire spectrum. 



To measure the intensity of any ray reflected from a pigment, a 

 paper was coated with it and placed adjacent to a white surface, and 

 it was so arranged that one shadow of the rod fell on the coloured 

 surface and the other on the white surface. The illuminations were 



