260 MESSRS. W. N. HARTLEY AND A. K. HUNTINGTON ON THE ACTION 
and iron. In some cases an amalgam containing silver, zinc, cadmium, sodium, 
aluminium, and magnesium has been used together with a point of iron. The spectrum 
most generally useful is that yielded by nickel points. In certain cases, particularly 
when several absorption bands occur, it is advisable to photograph with the slit open, 
so that the nickel lines may not be too distinct. 
The Photographic Process. 
The original method of photographing employed by Dr. Miller we have 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 iodised collodion ; secondly, a wet collodion process is disadvantageous when 
long exposure is sometimes necessary ; and thirdly, when 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 densest where the bath solution has most 
accumulated. Much larger and better spectra are obtained by using ordinary bromo- 
iodised collodion and an iron developer, but of course this process is rendered useless 
by the action of the ozone on the silver solution. 
The extraordinary improvements made of late years in the preparation and develop¬ 
ment of dry plates, together with the foregoing facts, combined to recommend a dry 
process. 
Successive trials have been made with plates coated with washed collodio-bromide 
emulsion, with the Beechy standard dry plates, uranium plates, Mr. Kennett’s and 
Wratten and Wainwright’s gelatine pellicle plates. The gelatine plates are to be 
preferred for two 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 a certain part of the 
spectrum than those coated with gelatine pellicle ; this may be seen by comparing the 
spectra of iron photographed on these two varieties of plates. One advantage of the 
gelatine plates is that they photograph more of the less refrangible rays than any 
others we have tried, and though the lengthening of the spectrum in consequence is 
comparatively slight, yet it is of importance in the examination of certain coloured 
substances, which while they transmit rays of higher refrangibility absorb the blue and 
violet. Such bodies are solutions of the nitrophenols and nitranilines. The exposure 
of the sensitive plates has been varied from 10 seconds to an hour and a-half, accord¬ 
ing to circumstances, depending partly on the plates employed and the object to 
be attained; but it is seldom that a longer period than a minute is necessary even 
for the production of negatives to print from, and when photographing absorption 
spectra with an open slit 10 seconds will suffice. 
