Photographing Ultra -Violet Sj^ectra. 103 



one inch in diameter, by slicing it longitudinally in two pieces, 

 and cutting off lengths of three-fourths of an inch. These are 

 fitted in metal frames and pieces of quartz are affixed to each end 

 by means of screws pressing upon a rim of metal. The cells are 

 made water-tight simply by grinding and polishing the ends, it is 

 not admissible to use any cement for the purpose. 



Description of the short focus or portable spectroscope-camera. 

 — The lenses of this instrument are two inches in diameter and 

 of fifteen inches focal length. The camera -body is of the same 

 construction as that already described, but the prism stand and 

 collimator tube, as well as the camera-back, are fitted on to a 

 small optical bench. This is intended to serve the purpose not 

 only of very careful and accurate adjustments, but to secure 

 rigidity so that the instrument may be transported from place to 

 place, and turned about in any position Avithout derangement of 

 its parts. The camera-body can be altogether removed and the 

 camera back turned round on its vertical axis by means of an 

 Archimedian screw beneath it. When once adjusted the screws can 

 be locked. It is, however, impossible to secure the apparatus 

 from an accidental wrench or blow, which might, in spite of the 

 security of the scrcAV adjustments alter the position of one or 

 other of the parts, it is therefore advisable to read off on scales 

 the exact angular position of the collimator tube, the prism, and 

 the camera. The minimum angle of deviation of the prism for 

 any particular ray is efiected by a screw movement. The slit 

 has jaws of iridium and a micrometer screw divided into fiftieths 

 of a millimeter. 



The camera body was made by Mr. Meagher, of Southampton- 

 row, London, the optical bench by Elliot Brothers, Strand, and 

 the slit, prisms, and lenses, by Mr. A. Hilger, Tottenham Court- 

 road, London. 



The Photographic Process. 



The original method of photographing employed by Dr. Miller 

 was found to be defective : first, because the more refrangible 

 end of the ultra-violet spectrum is extremely weak, if not entirely 

 wanting, when photographed on plates containing a plain iodized 

 collodion ; secondly, a wet collodion process is disadvantageous 

 when long exposure is sometimes necessary ; and thirdly, when 



