[From the ProcMdings of the Royal Society of Edinburgh, Session 1867-68.] 



YXXT. On the best Arrangement for producing a Pure Spectrum on a Scr- 



IN experiments on the spectrum, it is usual to employ a slit through 

 which the light is admitted, a prism to analyse the light, and one or more 

 lenses to bring the rays of each distinct kind to a distinct focus on the screen. 

 The most perfect arrangement is that adopted by M. Kirchhoff, in which two 

 achromatic lenses are used, one before and the other after the passage of the 

 light through the prism, so that every pencil consists of parallel rays while 

 passing through the prism. 



But when the observer has not achromatic lenses at his command, or when, 

 as in the case of the highly refrangible rays, or the rays of heat, he is re- 

 stricted in the use of materials, it may still be useful to be able to place 

 the lenses and prism in such a way as to bring the rays of all colours to 

 their foci at approximately the same distance from the prism. 



We shall first examine the effect of the prism in changing the conver- 

 gency or divergency of the pencils passing through it, and then that of the 

 lens, so that by combining the prism and the lens we may cause their defects 

 to disappear. 



When a pencil of light is refracted through a plane surface, its convergency 

 or divergency is less in that medium which has the greatest refractive index ; 

 and this change is greater as the angle of incidence is greater, and also as 

 the index of refraction is greater. 



When the pencil passes through a prism its convergency or divergency 

 will be diminished as it enters, and will be increased when it emerges, and 

 the emergent pencil will be more or less convergent or divergent than the 

 incident one, according as the angle of emergence is greater or less than that of 



