CONTRIBUTIONS TO SCIENCE. 477 



with any three portions of the pure spectrum formed by 

 light from E. The intervals between the sliders are closed 

 with hinged shutters, which allow the sliders to move with- 

 out 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, 6 inches long, and taper- 

 ing 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. The 

 gauge is balanced on one finger, and inserted into the slit 

 till the pressure just causes it to slide on the finger. This 

 (on the same principle as in Whitworth's Millionth Mea- 

 surer) ensures uniformity in the pressure in different 

 measurements. 



Now suppose light to enter at E, and to be refracted by 

 the two prisms P and P', and after reflection at S, to again 

 pass through the prisms. A pure spectrum, showing Fraun- 

 hofer's lines, is formed at A B, 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 refracted more or 

 less than the red, 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 correspond- 

 ing to the part of the spectrum which falls on the slit X 

 when white light is admitted at E. 



Let the slit Y correspond to another portion of the 

 spectrum, say the green : then, if white light be admitted at 



