23 THE CHEMISTRY OF THE SUN. [CHAP. 



purity of the spectrum was considerably impaired. It occurred 

 to Mr. Simms, 1 the celebrated optician, and independently to 

 Professor Swan, 2 that that difficulty and the difficulty of space 

 would be obviated if the incident pencil was converted into a 

 parallel beam before it entered the prism. This was effected by 

 the introduction of a second lens, called a " collimating " lens, 

 between the slit and the prism. This last improvement intro- 

 duced complete compactness, and as I said before, made the 

 instrument practically such as we now employ. 



Whilst describing the various steps which have led to the 

 perfection of the modern spectroscope, it is well to refer in this 

 place to an arrangement suggested by Janssen. The prism 

 employed in this arrangement differs from the simple one very 

 much as the achromatic telescope differs from the non-achro- 

 matic one. The object-glass of a telescope, as now constructed, 

 consists of two lenses made of different kinds of glass. Of course, 

 we have dispersion and deviation (terms already explained) 

 at work in both these kinds of glass, but the lenses are so 

 arranged, and their curves are so chosen, that, as a total result, 

 the deviation is kept while the dispersion is eliminated, so that, 

 in the telescope, we have a nearly white image of everything 

 which gives us ordinary light. So also in the spectroscope we 

 have an opportunity of varying the deviation and the dispersion. 

 By a converse arrangement we can keep the dispersion while we 

 lose the deviation ; in other words, we have what is called a 

 direct-vision spectroscope. If we take one prism composed of 

 glass which possesses considerable refractive power, and two 

 prisms of another kind which does not refract so strongly, and 



1 Memoirs R. A. S. 1839, vol. xi. pp. 168, 169. Mr. Simms in this paper (des- 

 cribing the measurement of the refractive index of the optical glass prepared by 

 the late Dr. Ritchie) states : " the only novelty of any consequence in this instru- 

 ment (the spectroscope employed) is the substitution of a collimator in place of a 

 slit in a window shutter." a 



2 Trans. Roy. Soc. Edin. 1847 and 1856. 



