660 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OF 
bring the invisible rays to a focus with the utmost facility. One great difficulty with 
which I have had to contend in photographing the diffraction spectrum has been to 
obtain a proper absorbing medium for the more refrangible end of the spectrum 
which would be placed in front of the slit without interfering with the invisible rays. 
In the prismatic as well as in diffraction spectrum this is of immense importance. 
With the former the prisms become illuminated with white light, and this causes a 
veil to appear with the image on the plate when developed, if the exposure be at all 
prolonged. With the latter, the ultra-reel of the first order overlaps the violet, blue, 
and green of the second order, so that it becomes a necessity to use an absorbing 
medium unless the different orders are separated by means of a prism : a proceeding 
which may introduce error in calculating the wave lengths, unless very great 
precautions are taken. 
In experimenting on this subject I employed the prismatic arrangement, and found 
that stained red and ruby glasses were both serviceable, as by interposing either of 
them I could obtain photographs of the lowest limit of the prismatic spectrum 
hitherto obtainable without them. Even when employing a glass cell containing an 
aqueous solution of potassium chromate, the same limit was reached. The times of 
exposure, however, differed considerably. Thus to obtain an impression of the ultra- 
red when the absorbing media were stained red and ruby glasses, the times of exposure 
were as one to two, and with the chromate half as much as that for the stained red. 
The latter was very suitable for photographs of the prismatic spectrum, but did not 
absorb sufficiently in the green to render it a useful adjunct with the diffraction 
spectrum. 
Messrs. Chance, of Birmingham, kindly supplied me with some microscopic glass 
flashed with ruby, and this has been of great service to me, as it cuts off all rays more 
refrangible than D. It is this which I have principally used in obtaining the diffrac¬ 
tion photographs, though a slightly lower limit is reached when separating the spectra 
by a rock-salt prism. Becent experiments have shown me that a solution of iodine in 
carbon disulphide, when placed in a rock-salt cell, may also be employed with advantage, 
as indicated by Tyndall. 
Map of the infra-red diffraction solar spectrum. 
In Plate 31 accompanying this paper I have endeavoured to give as accurate a map 
of the solar spectrum from A, as far as 10,750, as could be consistently obtained from 
the photographs I have taken with my large grating. A variety of photographs have 
been taken at different times, and it is by combining the best of these (and when I 
say the best I mean those which have the greatest definition for particular regions) 
that the map has the accuracy which I trust it will eventually prove it has. Mr. 
De La Bue kindly placed at my disposal, as often as I required it, the excellent 
micrometric measuring machine which he had constructed for measuring the series of 
