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XX. The Production of Monochromatic Light, or a Mixture 

 of Colours, on the Screen. By Capt. W. DE W. Abney, 

 R.E., F.R.S.* 



FOR some time past I have been making experiments on 

 the illuminating power of various sources of light, and 

 it became necessary in some investigations to produce fair- 

 sized patches of different monochromatic, and combinations of 

 monochromatic lights, upon a screen, all other colours being 

 absent. To obtain this result I had recourse to a modification 

 of Clerk-Maxwell's arrangement, as used for colour-mixing. 

 By his plan slits were inserted at different parts of the 

 spectrum as formed by an ordinary spectroscope. These slits 

 were then illuminated by light reflected from a w r hite screen, 

 and the prism was viewed through the slit of what is ordi- 

 narily the collimator. The prism was then seen to be coloured 



A 



with light from those rays which would 

 have fallen on those slits had the spec- 

 troscope been used in the ordinary man- 

 ner. In this arrangement only one 

 observer could be utilized at one time, 

 and my object was to allow several ob- 

 servers simultaneously to view the colour. 

 In order to effect this the following appa- 

 ratus was employed: — A collimator C, 

 the aperture of the lens L 2 of which 

 was 1^ inches, and the focal length 

 about 12 inches, was employed. On 

 the slit Si was cast an image of the 

 source of light A by means of the con- 

 densing lens Lj. This was of such an 

 aperture as entirely to fill the colli- 

 mating lens L 2 with the rays entering 

 through the slit. 



Two prisms, F x and P 2 , of 2^ inches by 

 If inch side, and of angles of 62°, gave 

 the necessary dispersion to the parallel 

 beam, from the collimator. The rays 

 were brought to a focus by means of the 

 lens L 3 , of about 14 inches focal length, 



* Communicated by the Physical Society : read Juiie 27, 1885. 



ii 



