SENSATIONS IN TERMS OF LUMINOSITY. 265 



slits were placed at Slit Scale Nos. (afterwards designated as SSN) 205 and 288 '5 

 (the latter number having been found by preliminary trials to be A), and a mixture 

 of the two beams fell on ad, making as near a match as possible with the colour of 

 the bichromate solution placed in the beam X. To the latter was added white from 

 the beam Z, and by altering the sectors and slits a perfect match could be obtained. 

 This being effected the width of the slits was measured by the method which has 

 been indicated in describing the apparatus. The small lens, Lg, was pushed in position 

 to the centre of the lens, L 4 , and the slits successively brought into the colour which 

 passed through the centres of these two lenses by sliding the slit holder along the 

 spectrum. A magnified and sharp image in monochromatic light was then thrown on 

 the scale below, dc, and the relative width of the apertures noted. This plan 

 obviates recourse to wedges for measuring, and is very convenient. It has been 

 employed by myself for measuring pin holes and other small apertures. The slits 

 having been measured they were replaced in position, and the luminosities measured 

 as has been described in ' Colour Photometry,' Part I.* Other matches were made 

 and the aperture of the slit again measured, but the luminosity not necessarily, as the 

 relation between width of slit and luminosity was determined from the first obser- 

 vations. Had the area of the retina on which the image fell been the same as that 

 employed in ' Colour Photometry,' Part III.,t and if the quality of the white had 

 not been slightly changed by the interposition of the bundle of glass, G" 1 , the 

 luminosities might have been taken from the tables in that paper. 

 The form of the equation then becomes of this kind 



(i.), 



where R, G, Or, W, and w stand for the red, the green, the orange, the added white, 

 and the white in the bichromate solution colour respectively, and m, n, a, b constants. 

 Now as the orange can contain but two sensations, and as the red is a pure 

 sensation and consequently not contaminated with white, it follows that a(iv) and 

 b (W) must be in nG, and we get 



m(B) + [n(G)-o(tir)-6(W)] = a(Or) ..... (ii.). 



That is, when the luminosity of the white is deducted from the luminosity of the 

 green we get the green sensation left, and, finally, we get 



a(Or) ........ (iii.). 



where US and GS are the red and green sensations respectively. 



* 'Phil. Trans.,' A, 1889. 

 t Ibid., 1896. 

 VOL. CXCIII. A. 2 M 



