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XIII. The Solar Spectrum, from X 7150 to X 10,000. 
By Captain W. de W. Abney, R.E., F.R.S. 
Received May 6,—Read May 21, 1885. 
[Plates 26-28.] 
The accompanying map of the solar spectrum between the limits of X 7150 and X 10,000 
is made from photographs taken with the diffraction gratings, and is more complete in 
every respect than the map from X 7600 to X 10,000 which appeared in the Phil. Trans, 
for 1880, under the title of “ The Method of Mapping the Least Refrangible End of 
the Spectrum.” In the map which accompanied the paper above referred to the scale 
numbers attached to the different lines have more accuracy than the wave numbers, 
and it was to correct the latter that the new series of photographs have been taken. 
It is my intention at some future and indeterminate time to publish the photographs 
of this region in connexion with Professor Rowland’s new photographic spectrum 
which he has in hand, and these will show the minute features of the spectrum down 
to a wave-length of nearly double that shown ; but as the wave-lengths adopted for 
the visible spectrum by Professor Rowland differ slightly from those given by 
o 
Angstrom, I have thought it better to publish the part which is to supersede the map 
of 1880 on the latter scale, leaving the discussion of the true wave-numbers to a later 
period. 
It must be recollected at the time the first map of this region was made that the 
photographic process employed was comparatively new—that is to say, it had been 
brought to its true state of perfection but a short time. Four years have elapsed 
since then, and much experimental work has been undertaken in connexion with it; 
and moreover the instrumental defects which were then present have been remedied 
to a large extent, new apparatus having been procured and finer gratings having been 
employed. 
Process employed. 
A reference to the paper of 1880 will give the idea of the sensitive salt of silver 
which I then employed, and as regards its nature I have not found any marked 
improvement to note ; but in the development I have succeeded in effecting an 
advance. My developer now, as then, is the ferrous oxalate developer, but I have 
found that by employing a glass plate which is previously coated with gelatine emul¬ 
sion instead of a bare glass plate to receive the sensitive salt, a more powerful 
solution of ferrous oxalate, without any large addition of restrainer, is capable of 
being used. The mode of preparation of the collodion emulsion is nearly the same as 
MDCCCLXXXVI. 3 N 
