Photogvaphinrj Ultra-Violet Sj^ectr a. 



90 



indices of quartz for the ordinary and extraordinary rays in the 

 ultra-violet spectrum. 



By the aid of a table which he gives it is possible to calculate 

 the position that a photographic plate should take in order that 

 any particular ray may be in focus. For future reference I quote 

 this table in full — 



Co-efficients necessary for the determination of the focal length of a quartz 

 lens for the idtra-violet rays of cadmium, its focal length for the ray 

 D being Icnoion. 



Numbers referring 

 to the lines of 



Numbers referring 

 to the lines of 



The focal length of the lens for the line D to be multiplied by the co- 

 efficient ojjposite the ray, the focal distance oftohich is reqtured to be 

 known. 



From this it may be ascertained that the focal distance for a 

 36-inch lens would, for the cadmiuia line 25, be 31 '^3 inches, and 

 for a 15-inch lens 13 inches. 



The angle, however, at which the plate must be placed to secure 

 a good photograph is a resultant of the refraction and dispersion of 

 the prism, and focal length of the lens. Of the three diactinic 

 materials of which I have had prisms constructed, Iceland spar 

 gives greatest dispersion, quartz next, and water least. In figures 

 A and B, Plate 15, is given a .sketch of the relative positions actually 

 assumed by the collimator tube and photographic plate when 

 accurately focussed, the optical train consisting of a pair of 15- 

 inch lenses, and one prism, with an angle of G()° made of water 

 and quartz respectively in the two cases. One water prism is 

 practically of no use for obtaining fine photographs, because the 

 rays strike the sensitised plate so obliquely by reason of their 

 running so nearly parallel to it that the images they produce 

 after passing into the sensitive film, instead of consisting of 

 sharply defined lines, take the forin of exceedingly narrow bands. 



