GG2 
CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OF 
wide dispersion of the grating, however, the width of the slit had but little had effect 
in giving the finest lines sharply defined ; in fact, the finest lines are the most sharply 
defined. 
I have already described the sensitive salt employed, so I need not refer to it further 
except to say that as the photographs were taken in the height of summer and autumn, 
the most sensitive salt could not be employed. 
I have not thought it expedient in the map to give the wave lengths as being 
absolutely definite; there may be, and probably is, a little uncertainty regarding them 
at the present time, but I hope at some future date to be able to make a table of 
absolute wave lengths which can be applied to the map. At any rate, it is believed 
that there cannot be any great error, since certain of the lines have been compared 
with the less refrangible end of the next order of the spectrum as photographed on 
the same plate, as described in the foregoing pages. It is worthy of remark that in 
this portion of the spectrum we come to a locality in which it is continuous. At first 
sight this seemed inexplicable, and many endeavours were made to obtain photo¬ 
graphs of such definition as to show absorption lines of some description. I have not 
succeeded in so doing, nor do I believe that they are to be found, except they be 
of the very finest description; in fact, much finer than any I have met with below 
A. In this conclusion I am borne out by a reference to the prismatic spectrum of 
this portion, in which the same locality exhibits a perfect blank as regards absorp¬ 
tion. From smaller photographs taken with the grating I believe it will be found 
that this region extends beyond wave length 12,000. It might be thought 
expedient that a reference should be made to the bands of lines which are due to 
atmospheric absorption. All that can be said is that, whether at a high altitude 
or a low one, the same intensities of lines exist, except, perhaps, in one or two cases 
which are very far from being well marked. I have therefore preferred to give all 
the lines present, leaving a discussion as to what they may be due to a subsequent 
paper. 
Of one thing, however, certainty may be expressed, viz.: that all rays will pass through 
several inches of glass and through half-an-inch of water; the absorption due to both 
these substances is evidently one gradually increasing as the wave length increases. I 
was not at all prepared to find that these long waves could traverse anything like the 
thickness of either one that they did without being affected. In some cases plates 
were absolutely exposed whilst immersed in an aqueous solution of potassium nitrite to 
prevent oxidation of the image, which can be caused by excess of light, as I have 
already shown in previous communications to the Royal Society, and still a low limit 
has been obtained. Regarding the time of exposure of the plates for the diffraction 
spectrum there is much variation; on a bright spring day the same action takes place 
in 10 minutes which in autumn takes half-an-hour to effect. I have found that when 
the A line was well visible with ruby glass in front of the slit a short exposure 
could be given. With the prismatic spectrum five minutes is sufficient exposure 
