234 Messrs. W. N. Hartley and A. K. Huntington [Jan. 9, 



scope of his invention which receives the ultra-violet rays upon a 

 fluorescent eye- piece, and so renders them visible. 



The Apparatus. — This consisted of a spectroscope attached to a pho- 

 tographic camera, the prism and lenses being of quartz. The electric 

 light consisted of sparks of great intensity passed between metallic 

 electrodes. To produce the sparks an induction coil, capable of giving 

 a 7-inch spark in air, was excited by five cells of Groves' battery. A 

 Leyden jar was interposed between the coil and the electrodes, each 

 surface of the foil measuring 72 square inches. The electrodes found 

 to answer best were points of nickel wire, containing a trace of 

 copper. Cells of glass, with quartz sides, were used for holding 

 liquids under examination. These cells were placed behind the slit of 

 the spectroscope, the spark passing in front ; volatile liquids were thus 

 prevented from taking fire, and a certain loss of light was avoided. 

 No condensing lens was used in front of the slit, because occasionally 

 it was found convenient to employ an amalgam, containing zinc, cad- 

 mium, aluminium, and magnesium, dissolved in mercury, in conjunc- 

 tion with a point of iron, and under these circumstances volatilised 

 mercury would condense on the lens. 



The Photographic Process. — It was found by experiment that a wet 

 collodion process, as used by Dr. Miller, was disadvantageous for 

 several reasons, and therefore dry plates were used. A preference 

 was given to gelatine pellicle plates, containing bromide of silver. 

 They are quite sufficiently sensitive, give a very finely defined picture, 

 and do not necessitate a varnishing process. The exposure has 

 generally been about 10 seconds, but on certain occasions plates have 

 been in the camera for an hour and a half. We have found no diffi- 

 culty in obtaining a constant stream of sparks, giving a steady light 

 for three-quarters of an hour without intermission. 



The Measurement of Absorption-Bands, 8fc. 



In order to measure the degree of absorption, exercised by different 

 substances the example of M. Soret has been followed, and the lines 

 of cadmium have been taken for the purpose. M. Mascart has 

 measured the wave-length of these lines both for the visible and the 

 ultra-violet rays. Sometimes measurements on the scale of wave- 

 length have been adopted, but in other cases it has been found more 

 convenient to make use of spectra as photographed. Photographs of 

 different metallic spectra employed are presented. The lines of cad- 

 mium are distinguished by the numbers assigned to them by M. Mas- 

 cart. A comparison is also given of the relative extent of the visible 

 and ultra-violet rays after passage through a prism. 



The prism was placed at the angle of minimum deviation for the 

 sodium line D. 



