ON THE THEORY OF COMPOUND COLOURS. 437 



Light from a sheet of paper illuminated by sunlight is admitted at the slits 

 X, Y, Z (fig. 8, Plate VII. p. 444), falls on the prisms P and P f (angles = 45), 

 then on a concave silvered glass, S, radius 34 inches. The light, after reflexion, 

 passes again through the prisms P t and P, and is reflected by a small mirror, 

 e, to the slit E, where the eye is placed to receive the light compounded of 

 the colours corresponding to the positions and breadths of the slits X, Y, and Z. 



At the same time, another portion of the light from the illuminated paper 

 enters the instrument at BC, is reflected at the mirror M, passes through the 

 lens L, is reflected at the mirror M', passes close to the edge of the prism P, 

 and is reflected along with the coloured light at e, to the eye-slit at E. 



In this way the compound colour is compared with a constant white light 

 in optical juxtaposition with it. The mirror M is made x of silvered glass, that 

 at M' is made of glass roughened and blackened at the back, to reduce the 

 intensity of the constant light to a convenient value for the experiments. 



This instrument gives a spectrum in which the lines are very distinct, 

 and the length of the spectrum from A to H is 3 '6 baches. The outside 

 measure of the box is 3 feet 6 inches, by 11 inches by 4 inches, and it can 

 be carried about, and set up in any position, without readjustment. It was 

 made by Messrs Smith and Ramage of Aberdeen. 



In obtaining observations from colour-blind persons, two slits only are 

 required to produce a mixture chromatically equivalent to white ; and at one 

 point of the spectrum the colour of the pure rays appears identical with white. 

 This point is near the line F, a little on the less refrangible side. From this 

 point to the more refrangible end of the spectrum appears to them " blue." 

 The colours on the less refrangible side appear to them all of the same quality, 

 but of different degrees of brightness ; and when any of them are made 

 sufficiently bright, they are called "yellow." It is convenient to use the term 

 "yellow" in speaking of the colours from red to green inclusive, since it will 

 be found that a dichromic person in speaking of red, green, orange, and brown, 

 refers to different degrees of brightness or purity of a single colour, and not 

 to different colours perceived by him. This colour we may agree to call 

 "yellow," though it is not probable that the sensation of it is like that of 

 yellow as perceived by us. 



Of the three standard colours which I formerly assumed, the red appears 

 to them "yellow," but so feeble that there is not enough in the whole red 

 division of the spectrum to form an equivalent to make up the standard white. 



