1 6 SECTION PHYSICS. 



prisms and the lens were made of ordinary optical glasses. Under these 

 conditions it is hardly possible to perceive rays more refrangible than N. 



It remained to be seen whether the apparatus would work satisfac- 

 torily if use were made of prisms and lenses of quartz or of Iceland spar, 

 neither of which absorb rays of great refrangibility. 



In one first arrangement, M. Soret followed the system of direct 

 vision employed by Herschel and Browning, the two prisms made 

 of Iceland spar being so cut that the crystallographic axis should 

 be parallel to the edges. The lenses are made of quartz. By this 

 apparatus the ultra-violet solar spectrum can be seen to the line R ; 

 but as much light is diffused (proceeding partly from extraordinary 

 rays, which are not included in the field of the spectrum) diaphragms 

 must be adapted to the tube for the observation to be easy. And 

 besides this, combination is somewhat difficult to carry out practi- 

 cally: it is necessary that the prisms should be extremely well cut 

 and adjusted with the greatest precision. 



The second arrangement simply consists in fitting a prism of Ice- 

 land spar, in which the edges are parallel to the axis, to an ordinary 

 spectroscope furnished with lenses of quartz. 



By concentrating the light of the sun upon the slit of this spectroscope 

 by means of a quartz lens with a long focus, the solar spectrum can 

 easily be distinguished as far as S ; if use is made of the light of the 

 Voltaic arc between different metals, the very refrangible brilliant lines 

 can be most easily perceived in it. Thus with cadmium all the lines 

 pointed out by Mr. Mascart can be seen up to the 25th. 



With regard to the nature of the fluorescent substance on which the 

 spectrum presents itself at the focus of the lens, a solution of esculine 

 in water seems to be the most favourable for the observation of the 

 part between H and O. For radiations of greater refrangibility, glass 

 of uranium is preferable. 



The spectroscope with a fluorescent ocular glass, without having, 

 perhaps, the precision of the photographic method, has nevertheless 

 great advantages for rapid observations. It allows of angular measure- 

 ment being taken, if, beforehand, two lines, in the shape of a cross, 

 have been drawn on the fluorescent plate, which would fulfil the func- 

 tions of a reticule. It can be used for a great number of researches. 

 MM. Soret and Sarasin have employed it for measuring the rotatory 



