their Relation to certain other Visual Phenomena. 269 



and the related wave-lengths are derived from a large-scale curve. 

 When it is desired to form a colour-patch consisting of a mixture of 

 light of given limiting wave-lengths, the slits in the slit-screen are 

 moved and adjusted until the limits of the bright bands seen in the 

 spectroscope coincide with the vertical cross-wire when the telescope is 

 set at the proper predetermined angles. 



Illumination and Luminosity. — It should be remarked that the colour 

 of an object, self-luminous or illuminated, is not completely specified 

 by a mere statement of the wave-lengths of the light which it emits or 

 reflects. This fact is of course well known, but it is doubtful whether 

 sufficient importance is always attached to it ; it has many times been 

 strikingly brought to my notice in the course of the experiments under 

 consideration. A complete account of the colour-conditions should 

 include a determination of the luminosity expressed in terms of some 

 standard unit; unfortunately, however, this cannot easily be given. 

 In order to furnish data for approximately estimating the luminosity 

 of the projected colour-patch when illuminated by selected spectral 

 rays, a rough photometric measurement was made of the illumination 

 of the white colour-patch produced by the whole recombined spectrum., 

 a " focus " electric lamp of 25*5 standard candle-power being employed 

 for the comparison. It was found that when the width of the colli- 

 mator-slit was 05 mm. (the width usually employed), the illumination 

 was equal to that due to 8800 standard candles at a distance of 1 metre, 

 or, as it may be called, to 8800 " candle-metres." Taking the lumi- 

 nosity-sensation due to this illumination as the unit or standard of 

 reference, the relative luminosity of a patch lighted by rays taken from 

 any parts of the spectrum can be deduced from Abney's luminosity- 

 curve for the normal electric-light spectrum.* Fov example, a purple 

 colour-patch was formed by combining the red between A. 8380 and 

 X 6600 with the blue-violet between X 4250 and X 4370. The area 

 enclosed by the curve and the ordinates meeting the horizontal axis at 

 6380 and 6600 was found to be 0-0361 of the whole, and the corre- 

 sponding area for the blue-violet 0*0027. The luminosity of the purple 

 patch relatively to that of a piece of white cardboard illuminated by 

 8800 candles at 1 metre was therefore 0-0361 + 0-0027 - 0-0388. The 

 variation from time to time of the intensity of the source of light, 

 though no doubt considerable, is for the present purpose unimportant. 



Approximate values for the illumination of the white disc due to 

 light reflected by the mirror C, fig. 3, and passing through the 

 apertures in the diaphragm O, are given in the following table. 



* ' Phil. Trans./ A, vol. 193 (1899), p. 282. 



