20 REPORT—1874, 
highly bromized collodion and strong alkaline development. The author gave an 
account of the advantages thus secured, and discussed how each could be best 
obtained, especially alluding to the phenomenon of irradiation. He mentioned 
that at each station the photographic party would consist of one officer and three 
sappers all well trained to the process, so that all excitement at the critical 
moment might be avoided. Practice on the mock transit at Greenwich has given 
them a thorough knowledge of each phase of the phenomenon. 
On the Spectrum of Coggia’s Comet. By W. Hueerns, D.C.L., FR. 
Preliminary Note on Coggia’s Comet. By J. N. Lockyer, F.B.S, 
Preliminary Note on a New Map of the Solar Spectrum. 
By J. N. Locxyzr, F.R.S, 
On Photography in Connexion with Astronomy. By Col. Srvarr Worrzey. 
Having been asked by the Astronomer Royal for some information in connexion 
with photographic processes with a view to their being used for the transit of Venus, 
and having had the advantage of discussion of the various processes with my friend 
Captain Abney, who has been in charge of the transit photographic work, I made 
at various times during the past summer a number of experiments in solar photo- 
graphy, the result of which may possibly be of use to future workers in this branch 
of science. 
Taking the ordinary commercial collodions manufactured for photographie pro- 
cesses, we are at once struck with the difficulty of getting accurate micrometrical 
measurements, in consequence of the varying amount of contraction and expansion 
possessed by the collodion-film in its wet and dry states. To counteract this diffi- 
culty it is necessary to make the pyroxyline with the maximum of water added to 
the acids which they: will bear without dissolving the cotton when immersed 
therein ; and it is also desirable to reduce considerably the proportion of nitric acid 
to sulphuric, with the object of obtaining a film which shall neither contract nor 
expand when either wet or dry. By using such a pyroxyline I have been enabled 
to make a film on which the most delicate micrometrical measurements can be 
registered with absolute perfection. 
The next point to be considered in connexion with astronomical photography is 
the radiation and halation produced where the bright and dark parts of the picture 
meet. This appears to proceed from two different causes—one, which we may 
call “halation,” being the reflection of light back from the glass which supports 
the film into the film itself; and the other, which I will call “ radiation,” appears 
to be an action in the sensitive molecules of the film itself, and occurs no matter 
on what support the film may be laid. Halation from the first of these two causes 
can be prevented in two ways—one method being to place a dark pigment on the 
back of the glass plate and in optical contact therewith ; and the other, and far 
preferable one, to stain or dye the film itself with such an amount of orange or red 
colour as shall stop the rays of light from getting down to the glassand being thence 
reflected. 
But the radiation which takes place in the film itself is much more difficult to 
subdue, cannot be subdued by mechanical means, and can only be subdued by the 
use of certain chemicals in the film, differing somewhat from those in use in the 
ordinary photographic processes. y 
Before pointing out what I consider the best means of avoiding this injurious 
halation I will point out what I consider it to be, and why I consider it to be so. 
I think the radiation in photographic films, which is so unpleasantly apparent when 
a bright object is photographed in close proximity toa dark one, is due to what I ma: 
call a “creeping ” of the superabundant light, which has done its work on the bright 
