422 ON THE THEORY OF COMPOUND COLOURS. 



and Y simultaneously, the colour seen at E will be a compound of red and green, 

 the proportions depending on the breadth of the slits and the intensity of the 

 light which enters them. The third slit Z, enables us to combine any three kinds 

 of light in any given proportions, so that an eye at E shall see the face of the 

 prism at P uniformly illuminated with the colour resulting from the combination 

 of the three. The position of these three rays in the spectrum is found by 

 admitting the light at E, and comparing the position of the slits with the 

 position of the principal fixed lines ; and the breadth of the slits is determined 

 by means of the wedge. 



At the same time white light is admitted through BC to the mirror of black 

 glass at M, whence it is reflected to E, past the edge of the prism at P, so that 

 the eye at E sees through the lens a field consisting of two portions, separated 

 by the edge of the prism; that on the left hand being compounded of three 

 colours of the spectrum refracted by the prism, while that on the right hand is 

 white light reflected from the mirror. By adjusting the slits properly, these two 

 portions of the field may be made equal, both in colour and brightness, so that 

 the edge of the prism becomes almost invisible. 



In making experiments, the instrument was placed on a table in a room 

 moderately lighted, with the end AB turned towards a large board covered with 

 white paper, and placed in the open air, so as to be uniformly illuminated by 

 the sun. In this way the three slits and the mirror M were all illuminated 

 with white light of the same intensity, and all were affected in the same ratio 

 by any change of illumination ; so that if the two halves of the field were 

 rendered equal when the sun was under a cloud, they were found nearly correct 

 when the sun again appeared. No experiments, however, were considered good 

 unless the sun remained uniformly bright during the whole series of experiments. 



After each set of experiments light was admitted at E, and the position of 

 the fixed lines D and F of the spectrum was read off on the scale at AB. It 

 was found that after the instrument had been some time in use these positions 

 were invariable, shewing that the eye-hole, the prisms, and the scale might be 

 considered as rigidly connected. 



