240 Intelligence and Miscellaneous Articles. 



disks of black and white cardboard being simultaneously fastened 

 on the same axis, so that by varying the relative proportions of the 

 latter, a series of greys could be produced at will. 



The compound black-and-white disk was now arranged so as to 

 furnish by rotation a grey which was decidedly darker than the 

 vermilion ; this grey tint was then gradually lightened till the ob- 

 server became doubtful as to the relative luminosities of the red 

 and grey disks ; the angle occupied by the white sector was then 

 measured. Next, a grey decidedly more luminous than the vermi- 

 lion was compared with it, and diminished in brightness till the 

 observer again became doubtful, when a second measurement was 

 taken. All this time the manipulation was performed by an assist- 

 ant, the experimenter giving directions, but remaining in ignorance 

 of the results to the end. The mean of ten such experiments 

 assigned to the vermilion disk a luminosity of 23*8, that of white 

 cardboard being taken as 100. In this experiment and in all those 

 that follow, proper corrections were made for the amount of white 

 light reflected by the black disk, this having been previously ascer- 

 tained in a manner which will be described in a future communi- 

 cation. 



In order to test the correctness of the final result, the luminosity 

 of a blue-green disk, correctly complementary in colour to the ver- 

 milion, was next measured in the same way : it proved to be 26-56. 

 The vermilion and blue-green disks were then combined, according 

 to Maxwell's method, so as to obtain a pure grey by rotation, and 

 the angular proportions of these coloured surfaces and the value of 

 the grey in terms of white and black cardboard measured. The 

 grey thus obtained had a luminosity of 24-54, that of white card- 

 board being 100. Next, the value of this same grey was calculated 

 from the measured luminosities of the two coloured disks, and the 

 proportions of these colours required to produce a pure grey by 

 mixture on the rotation-apparatus ; the calculated value was 25-47. 



This agreement proves the correctness of the photometric com- 

 parison, and also of Grassman's assumption that the total intensity 

 of the mixture of masses of differently coloured light is equal to 

 the sum of the intensities of the separate components, which, so 

 far as I know, has not before received an experimental confirmation. 



Corresponding measurements were made with a green and its 

 complementary purple disk ; also with a blue and its complementary 

 yellow disk. The results are given below. 



T . ., Grev Grey 



Luminosity. (obser ^ ed) . (calculated). 



SSK ::::::::&} 24 ' 54 25 ' 47 

 aasSr.::-::!™} 54 " 51 53 - 92 



Green 41-19 



Purple 14-83 



— Silliman's American Journal, February 1878. 



Purple 14-83 [ 24 ' 94 26 - 26 



