ON THE THEOKY OF COMPOUND COLOURS. 421 



At E is a fine vertical slit ; L is a lens ; at P there are two equilateral 

 prisms. The slit E, the lens L, and the prisms P are so adjusted, that when 

 light is admitted at E a pure spectrum is formed at AB, the extremity of the 

 long box. A mirror at M is also adjusted so as to reflect the light from E 

 along the narrow compartment of the long box to EG. See Fig. 3. 



At AB is placed the contrivance shewn in Fig. 2, Plate I. A'B' is a rect- 

 angular frame of brass, having a rectangular aperture of 6 x 1 inches. On this 

 frame are placed six brass sliders, X, Y, Z. Each of these carries a knife-edge 

 of brass in the plane of the surface of the frame. 



These six moveable knife-edges form three slits, X, Y, Z, which may be 

 so adjusted as to coincide with any three portions of the pure spectrum formed 

 by light from E. The intervals behind the sliders are closdd by hinged shutters, 

 which allow the sliders to move without letting light pass between them. 



The inner edge of the brass frame is graduated to twentieths of an inch, 

 so that the position of any slit can be read off. The breadth of the slit is 

 ascertained by means of a wedge-shaped piece of metal, six inches long, and 

 tapering to a point from a breadth of half an inch. This is gently inserted into 

 each slit, and the breadth is determined by the distance to which it enters, the 

 divisions on the wedge corresponding to the 200th of an inch difference in 

 breadth, so that the unit of breadth is '005 inch. 



Now suppose light to enter at E, to pass through the lens, and to be 

 refracted by the two prisms at P ; a pure spectrum, shewing Fraunhofer's lines, 

 is formed at AB, but only that part is allowed to pass which falls on the three 

 slits X, Y, Z. The rest is stopped by the shutters. Suppose that the portion 

 falling on X belongs to the red part of the spectrum ; then, of the white light 

 entering at E, only the red will come through the slit X. If we were to admit 

 red light at X it would be refracted to E, by the principle in Optics, that the 

 course of any ray may be reversed. If, instead of red light, we were to admit 

 white light at X, still only red light would come to E ; for all other light 

 would be either more or less refracted, and would not reach the slit at E. 

 Applying the eye at the slit E, we should see the prism P uniformly illuminated 

 with red light, of the kind corresponding to the part of the spectrum which 

 falls on the slit X when light is admitted at E. 



Let the slit Y correspond to another portion of the spectrum, say the green ; 

 then, if white light is admitted at Y, the prism, as seen by an eye at E, will 

 be uniformly illuminated with green light ; and if white light be admitted at X 



