664 CAPTAIN W. DE W. ABNEY ON THE PHOTOGRAPHIC METHOD OF 
Taking G, F, E, and D as fiducial points as given by Lamansky, it was found that 
the lines joining them lay nearly in a straight line, with the exception of that joining 
F and G. The lines F, E, and D were taken as correct, and the straight line on which 
those points most nearly lay was prolonged to meet the line on which the abscissae 
were measured. The various bands and lines of absorption in the ultra-red portion of 
the spectrum, as shown by the photograph, were inserted. It will be seen (Plate 32, 
fig. 1) that the thermal minima agree with 2 , n, and r, and the maxima with X, cr, 
and tf>, positions which would be naturally assumed as most probable. There cannot 
be much doubt as to the correctness of this view, if a reference be made to a passage in 
Lamansky’s paper. He says: “ These three breaks or bands are not of equal breadth ; 
the first is much more sharply separated from the second than the second from the 
third. It may easily happen if the movement of the thermo apparatus be not suffi¬ 
ciently delicate that the second and third appear as one common break .” A glance at 
the diagram will show that this is most probable; i r, cr, and r ivoidcl easily blend into 
one. Lamansky asks the question, “Is not the limit of refraction situated at the 
place where the heat effect of the solar spectrum attains its last maximum?” If the 
positions assigned to these maxima by myself be correct it is evident that it has not 
been attained. From a careful perusal of Sir John Herschel’s memoir, which is to 
be referred to, it would seem that the maxima show themselves by his method with 
greater di ffi culty the nearer they approach the limit of refraction. 
Herschel’s thermograph. 
In the Philosophical Transactions for 1840 is to be found the thermograph of the 
prismatic spectrum as delineated by Sir John IJerschel. The thermograph itself was 
made by causing an image of the solar disc to focus itself on a sheet of blackened 
tissue paper moistened with alcohol, after passing through a flint or other prism ; the 
drying of the alcohol in some parts more rapidly than in others gave a figure such as 
shown in Plate 32, fig. 4, demonstrating that the heating effect of the spectrum was 
discontinuous. Figs. 3, 4, and 5 are taken from Sir John Herschel’s paper, and it 
should be noted that these are the images obtained by the sun’s whole disc, whilst the 
photograph was taken with a narrow slit. 
Lord Rayleigh gives an account of a repetition* of Herschel’s experiments, and 
finds that the thermographs he obtained are not comparable with Herschel’s, the 
maxima of heating effect lying in very different positions; he also indicated that the 
spot e, and probably §, lay beyond the theoretical limit of the prismatic spectrum. In 
some of my own experiments with the same method I found that I obtained thermo¬ 
graphs which were very similar to those of Herschel with the exception of the 
spot e— a spot, it may be remarked, the existence of which Sir John Herschel himself 
did not absolutely insist upon. I have tried to examine the latter’s thermograph with 
* Phil. Mag., vol. iv. (fifth series). 
