144 EXPERIMENTS OX COLOUR, AS PERCEIVED BY THE EYK. 



(3) Union of Coloured Beams by a Prism so as to form one Beam. 



The mode of viewing the beam of light directly, without first throwing it 

 on a screen, was not much used by the older experimenters, but it possesses 

 the advantage of saving much light, and admits of examining the rays before 

 they have been stopped in aiiy way. In Newton's llth proposition of the 2nd 

 Book, an experiment is described, in which a beam is analysed by a prism, 

 concentrated by a lens, and recombined by another prism, so as to form a beam 

 of white light similar to the incident beam. By stopping the coloured rays at 

 the lens, any proposed combination may be made to pass into the emergent 

 beam, where it may be received directly by the eye, or on a screen, at pleasure. 



The experiments of Helmholtz on the colours of the spectrum were made 

 with the ordinary apparatus for directly viewing the pure spectrum, two oblique 

 slits crossing one another being employed to admit the light instead of one 

 vertical slit. Two pure spectra were then seen crossing each other, and so 

 exhibiting at once a large number of combinations. The proportions of these 

 combinations were altered by varying the inclination of the slits to the plane of 

 refraction, and in this way a number of very remarkable results were obtained, 

 for which see his Memoir, before referred to. 



In experiments of the same kind made by myself in August 1852 (inde- 

 pendently of M. Helmholtz), I used a combination of three moveable vertical 

 slits to admit the light, instead of two cross slits, and observed the compound 

 ray through a slit made in a screen on which the pure spectrum is formed. 

 In this way a considerable field of view was filled with the mixed light, and 

 might be compared with another part of the field illuminated by light proceeding 

 from a second system of slits, placed below the first set. The general character 

 of the results agreed with those of M. Helmholtz. The chief difficulties seemed 

 to arise from the defects of the optical apparatus of my own eye, which ren- 

 dered apparent the compound nature of the light, by analysing it as a prism 

 or an ordinary lens would do, whenever the lights mixed differed much in 

 refrangibility. 



