106 ScientifiG Proceedings, Royal Dublin Society. 



When a deposit of silver has been made in an exposed fihn 

 by immersion in ferrous oxalate solution, particles of silver 

 bromide are reduced in increasing proportion with the length of 

 time that the plate is left in the developing solution. The group 

 of air lines at the less refrangible end is, as a rule, the most active 

 portion of a spectrum. Hence, when the exposed plate is put 

 into the developer, they appear sometimes half a minute at least 

 before the more refrangible rays are visible. They get, therefore, 

 half a minute start — so to speak — of the rest of the spectrum 

 and the larger the deposit of silver upon them the greater will 

 be the rate of reduction of fresh particles. "With the pyrogallic 

 developer the action is different ; there is not a progressive rate of 

 reduction of silver bromide, increasing with the length of period 

 of immersion in the solution ; hence, though the less refrangible 

 rays are first rendered visible, their development soon ceases, and 

 that of the other portions of the plate which have been more 

 slowly acted on become developed to an extent which differs but 

 little from that of the air lines. 



After the development of the plates they are washed and 

 steeped for some time in a solution of alum. Subsequent and 

 final washing is carried on in a stream of water for several hours. 

 Very thorough washing should be regarded as essential, otherwise 

 gelatine negatives are apt, after a period varying from three 

 months to two years, to become yellow or to fade. Negatives are 

 the better for a coat of varnish, which may be a hard photographic 

 varnish diluted with once to twice its volume of strong alcohol. 



The Optical Train. 

 It is a remarkable fact, as pointed out by Miller, that all the 

 materials which enter into the composition of glass are of much 

 greater transparency to the photographic rays than glass itself. 

 As for instance quartz, Iceland spar, and rock-salt ; likewise, also, 

 is alum very diactinic. The small amount of iron which is found in 

 specimens of crown glass was supposed to be the cause of the difference. 

 In the hopes of finding a glass nearly, if not quite as diactinic as 

 quartz, I have at various times examined samples of great purity 

 made by Feil, in Paris, and others, but in no case have I met 

 with a material suitable for the examination of ultra-violet 

 spectra. The disadvantage attending the use of quartz and 



