MAPPING THE LEAST REFRANGIBLE END OF THE SOLAR SPECTRUM. 663 
where four or five prisms of flint glass are used and the slit very narrow, the focal 
length of the mirror being about 30 inches. 
Map of the infra-red prismatic solar spectrum. 
Plate 30, fig. 2, is a drawing from a photograph of the prismatic spectrum,* showing- 
bands of absorption, but which resolve themselves into lines when the diffraction 
grating is used. Excepting the bands A, Z, and X, these bands at first are some¬ 
what difficult to recognise in the diffraction spectrum, but by using an artifice they are 
at once distinguished. If the diffraction photograph be held obliquely from the eye 
in such a manner that the lines appear to blend one into the other, we have the same 
appearance of bands as in the prismatic spectrum, f In my paper originally sent in to 
the Loyal Society a remark was incidentally made that the limit of this spectrum was 
apparently reached; but Professor Stokes pointed out that the supposition was 
incorrect, as by setting up a curve of — and producing it, there could be but little 
doubt that the theoretical limit lay beyond the boundary of the photograph. In the 
diagram \ this latter curve has been drawn, and also a curve showing absolute wave 
lengths. 
Lamansky’s thermograph. 
In December, 1871, Lamansky published in the c Monatsberichte der Konigl. 
Akademie der Wissenschaften zu Berlin,’ a communication on the heat spectrum of the 
sun and the lime light, to the original of which I have not had access, but have used a 
translation which appears in the Philosophical Magazine for April, 1872. Lamansky’s 
thermograph was made by aid of a thermopile, the deflections being a measure of the 
heating effect on lamp-black of the various parts of the spectrum. In some cases he 
used a flint-glass prism with which to obtain his spectrum, in others he used a rock- 
salt prism, and it is from the results with the latter that his diagram is constructed. 
In making a comparison of this thermograph§ with the photograph of the prismatic 
spectrum I have had recourse to the method of graphically setting up ordinates 
on the diagram. || 
* The bands y, p t/ were marked from photographs taken on March 25, 1880, a note to that effect 
having been sent to the Secretary of the Royal Society June 10, 1880. 
f Photographs taken with a wide slit also give the same effect. 
t The amended diagram and description was communicated to the Royal Society January 8, 1880. 
§ The first comparison was withdrawn from the paper, owing to the construction of a new diagram 
showing the curve of wave-lengths and — as suggested by Professor Stokes. 
\ ^ 
|| In the copy of Lamansky’s diagram in the Philosophical Magazine there is evidently an error, in 
that the ordinate of the last maximum cuts the descending curve, This has been corrected. 
4 Q 2 
