216 Mr. T. E. Doubt on the Measurement of Colour 



unknown spectrum, and the D lines having been brought 

 into coincidence the procedure indicated above may be pro- 

 ceeded with. 



For eye-observations the most convenient arrangement 

 would be to place a small plate of optically worked glass 

 between the reference prism, generally provided with a spec- 

 trometer, and the slit, a simple arrangement serving to adjust 

 the adjacent surfaces (which should be silvered) for 

 parallelism. 



It will be seen that the phase-changes produced by the 

 silver do not introduce any serious errors into the final re- 

 sults. Wiener* has shown that for light reflected from a 

 silver film of sufficient thickness the phase-change is very 

 nearly independent of the wave-length. To further test this 

 a streak of silver was rubbed off the glass plate which is 

 placed next to the collimator slit, and a photograph of the 

 spectral bands obtained. The displacement of the bands, 

 where the light had been reflected from the silver, relatively 

 to the bands formed where the light had been reflected from 

 the clear glass, was practically constant for the whole length 

 of the spectrum. 



In conclusion, we think that it may be claimed that by 

 means of this application of a well-known principle to spec- 

 troscopy, the difficulties incident to the reduction of prismatic 

 spectra in terms of wave-lengths or frequencies are greatly 

 reduced, the whole process as above described requiring no 

 special experience in the experimenter. 



The experimental work incidental to this investigation has 

 been performed partly at the Davy-Faraday Laboratory, 

 Royal Institution, and partly at the Royal College of Science, 

 South Kensington. For the facilities afforded us individually 

 at these institutions our joint thanks are due. 



XVI. Note on the Measurement of Colour and the Determi- 

 nation of White Light. By T. E. Doubt, University of 

 Washington, Seattle, Washington f. 



IN his paper on the " Theory of Compound Colour " Maxwell 

 has given colour-equations to represent white light. From 

 these equations, by eliminating the quantity W between any 

 two equations, the relation between any three colours may be 

 obtained. In making his determinations he used a white 

 diffusing-screen illuminated by direct sunlight. The quality 

 of the light that is reflected by a screen depends somewhat 



* O. Wiener, Wien. Ann. xxxi. p. 629 (1887). 

 t Communicated bv the Author. 



