104! Scientific Proceedlnr/s, Royal Duhlin Society. 



working in a small room the ozone generated by the electric 

 discharge acts upon wet collodion plates in such a manner that 

 they become coated with a thick deposit of silver directly the 

 developing solution is applied, the deposit being the densest 

 where the bath solution has most accumulated. Much longer 

 and better spectra are detained by using ordinary bromo-iodized 

 collodion and an iron developer, but of course this process is 

 rendered useless by the action of the ozone on the silver 

 solution. 



The extraordinaiy improvements made of late years in the 

 preparation and development of dry plates, together with the 

 foreo-oing facts, combined to recommend a dry process. 



Successive trials have been made with plates coated with 

 washed collodio-bromide emulsion, with the ]lev. Canon Beechy's 

 collodion emulsion plates, and with gelatine pellicle plates. The 

 o-elatine plates are to be preferred for tAvo reasons; if they are 

 wanted for the production of negatives to print from, the film is 

 exceedingly fine and even in texture, and on the other hand, if 

 transparencies showing absorption bands are desired they need 

 not be varnished. The Beechy plates are more sensitive to very 

 feeble rays in the more refrangible part of the spectrum than 

 those coated with gelatine pellicle ; this may be seen by compar- 

 ino- the spectra of iron photographed on these two varieties of 

 plates. One advantage of the gelatine plates is that they photo- 

 graph more of the less refrangible rays than any others, and 

 thouo-h the lengthening of the spectrum in consequence is com- 

 paratively slight, yet it is of importance in the examination of 

 certain coloured substances, which while they transmit rays of 

 hio-her refrangibility absorb the blue and violet. Such bodies 

 are solutions of the nitrophenols and nitranilines. The exposure 

 of the sensitive plates has varied with the long-focus camera 

 from three seconds to an hour and a half, according to circum- 

 stances, depending partly on the plates employed and the object 

 to be attained ; but it is seldom that a longer period than one or 

 two minutes is necessary even for the production of negatives to 

 print from, and when photographing absorption spectra with an 

 open slit three seconds will suffice. The exposure is four times 

 as rapid with the portable camera with lenses of fifteen inch 

 focus. 



