IN CONNEXION WITH THE SPECTRUM OP THE SUN. 
571 
whole length of spectrum capable of being photographed, before the conditions set 
forth at the commencement of this paper are fulfilled. Experiments made in the 
course of the present work show that, with a bromo-iodized collodion, it is possible to 
photograph as far as E in the green (wave-length 5269), with exposure of from two 
to three minutes. Neither can there be much doubt that, in a short time, we shall 
be in possession of methods enabling us to photograph any part of the spectrum with 
equal facility. 
The importance of the photographic method will be still further enforced, if we con¬ 
sider that the maps of metallic spectra at present in existence, although constructed 
with the greatest skill, are necessarily incomplete, and are, moreover, not free from 
impurity lines. Before we can hope to arrive at any great generalisation in the theory 
of the spectrum, it is obvious that we must have pure spectra to deal with ; and, as I 
have previously shown, the photographic method is the only one which enables such a 
purification to be effected. 
V. PHOTOGRAPHIC PROCESS EMPLOYED. 
The silver bath employed contains 40 grains of silver nitrate to the ounce of 
distilled water. It is made as neutral as possible. 
Amongst many developers which were tried, I have found that used by Cornu to be 
best for these investigations, viz. :— 
1 litre.Distilled water. 
60 cubic centimetres . . Saturated solution of ferrous sulphate. 
30 ,, ,, . . Glacial acetic acid. 
30 „ ,, . . Alcohol. 
In course of these researches I have used many collodions, but Blanchard’s extra 
bromo-iodized is by far the best and most suitable for spectrum photography, as its 
power for imparting detail is greater than any other, and the granular appearance 
under magnification is less than that produced by others, although this defect is still 
great, and leaves much to be desired. 
The emulsion process and also the dry plate processes have yet to be more thoroughly 
tested in connexion with this work, but at present the time of exposure required has 
proved a fatal objection. 
Having obtained the negative, whether from the grating or a train of prisms, the 
next process is to produce the necessary enlargement. I have found no lens so well 
suited for these enlargements as Mr. Dallmeyer’s rapid rectilinear. The working- 
positive employed is a little below the scale of the map. The exposure required for 
transparencies is not so long as for a negative by about one-tliird. The intensifying 
solution should be passed over once, merely to secure the fine lines. The cyanide 
solution should be made rather weak for this work. 
4 E 2 
