Intensity ofLiglit reelected from Coloured Surfaces. 63 



coloured sector, and of the black and white sectors, it was 

 found that the luminosity curves of the colours were correct. 



This is an indirect, though very accurate, method of 

 ascertaining the luminosity of a coloured surface, and re- 

 quires more time than can often be spared with that object 

 in view. Professor 0. Rood, in his " Modern Chromatics," 

 indicates the manner in which he proceeds, which is as follows. 

 He finds '' that with the aid of rotating disks the second con- 

 stant of colour (luminosity) can often be determined. Let us 

 suppose we wish to determine the luminosity of paper painted 

 with vermilion : a circular disk about six inches in diameter is 

 cut from the paper and placed on a rotation apparatus . . On 

 the same axis is fastened a double disk of black and of white 

 paper, so arranged that the proportions of black and white 

 can be varied at will. When the whole is set in rapid rota- 

 tion the colour of the vermilion paper will of course not be 

 altered, but the black and white will blend into a grey. This 

 grey can be altered in its brightness till it seems about as 

 luminous as the red." He then proceeds to give examples. 

 This method must not only be difficult to manage, but also 

 must be tedious before the match can be determined. A 

 method based on the same plan is given later in this paper, 

 which makes the matching of the luminosities more easy. 



The following plan, however, I venture to think is much 

 simpler and more certain in result, and is essentially founded on 

 the method which General Festing and myself adopted in mea- 

 suring the luminosity of the spectrum itself. In that we found 

 that any coloured light might be compared with any other or with 

 white light by rapidly changing the luminosity of one colour, 

 when the two were in juxtaposition, making it first decidedly 

 too fight and then too dark, and then gradually diminishing 

 the oscillations until an equality of luminosity was obtained. 

 The two colours were placed alongside one another, it may 

 be recollected, by means of the Rumford method of shadows. 

 In the case of the spectrum the rapid diminution in lumi- 

 nosity from the yellow to each end of the spectrum enabled 

 the change of luminosity to be quickly made by sliding the 

 card containing the slit (which allowed a slice of coloured 

 light to pass and to subsequently form a patch of that colour 

 on the screen) along the spectrum on each side of the maxi- 

 mum, and then noting the position of the colour, which 

 balanced a white light of know^n intensity. In the case of 

 the measurement of the luminosity of coloured paper, this 

 method evidently was inapplicable, and it remained to devise 

 some other. 



In our former experiments of the measurement of light 



