Fhotographing Ultra-Violet Spectra. 107 



Iceland spar arises from the difficulty in getting rid of the effects 

 caused hy double refraction. 



Beautiful results can be obtained by the use of hollow prisms 

 filled with water, their sides being made of thin plates of quartz ; 

 but it is necessary to have more than one prism, otherwise the 

 lateral dispersion is not sufficient to enable one to focus accurately 

 more than a very limited portion of the spectrum. 



The prisms which I am now using are of quartz, and corrected 

 in a manner for double refraction, which was kindly communi- 

 cated to me by letter by M. Cornu. The double refraction which 

 is noticed as aflecting the lines of metallic spectra as photo- 

 graphed, is caused by circular polarization, causing the ray to be 

 turned to the right or left. To obviate this each prism, with an 

 angle of 60°, is composed of two equal halves — one cut from right- 

 handed, the other from left-handed, quartz. Optical contact 

 between the faces is secured by means of a thin layer of glycerine 

 or of pure water. The lenses, in the same way, are cut to correct 

 each other. In the case of those of long focus — G3 inches — they 

 are both doubly convex, but the short focussed lenses are plane 

 convex. 



In photographing absorption spectra the slit of the instrument 

 is wide open, but of course this is inadmissible in the case of 

 line spectra. The width of the slit is then narrowed to -5^oth of 

 an inch or thereabouts, and as some difficulty is experienced 

 in keeping it free from dust, it has been found convenient to 

 cover it with a thin plate of quartz, which can of course be kept 

 clean by simply wiping it with a leather. 



