of Heat in the Prismatic Spectrum. 34 1 



phuric acid, the maximum of heat seemed to fall between the 

 yellow and the red. I must however observe, that the experi- 

 ments with the two liquids last named belonged to those which 

 were made last, and were therefore not so frequently repeated 

 as might have been necessary, not having been made under 

 the most favourable circumstances. With sulphuric acid this 

 was the only experiment. 



In the preceding experiments the prisms had generally the 

 normal position; and their refracting angles being always turned 

 downwards, the spectra had, at least at the beginning of every 

 series of experiments, the lowest station. 



But as (as has been observed) the prismatic colours were 

 sometimes carried over the thermoscope by turning the prism 

 round on its axis, and therefore as considerable deviations 

 from the above position occasionally took place, it was ne- 

 cessary to know what influence this circumstance might have 

 had on the results, and in fact whether the effect on the ther- 

 moscope was varied by opposite positions, viz. with the greatest 

 and smallest angle of incidence of the light ; by which, in both 

 cases, the spectrum would be placed higher,with this difference, 

 that in the former case it would be more compressed, and in 

 the latter more enlarged, than in the normal position. 



For this purpose I made comparative experiments with 

 prisms of flint and crown glass (Experiments 44 and 45). I 

 observed that the heat in the spectrum decreases when the in- 

 cident angle is greater or smaller than it is in the normal posi- 

 tion ; but the place of the greatest heat is not changed, falling 

 in the flint-glass always beyond the limit of the red. The 

 same experiments were made with prisms of Bohemian glass 

 (Nos. 1 and 4), and with a greater angle of incidence, when 

 the px-oportion remained the same as in the normal position, 

 the heat in the red being greater than below the red, but the dif- 

 ference being much less: whilst with smaller angles it was the 

 same in both, and one inch below the red, greater than in the 

 green. The experiments therefore with these two prisms, re- 

 sembling crown-glass, also confirm the above axiom, — that the 

 heat is always greatest in the same prismatic colours in the 

 normal position, and is reduced in proportion to the distance 

 of the prism from that position. But then the brilliancy of 

 the colours and the intensity of the light in general is also 

 much diminished, which ought well to be borne in mind. The 

 experiments also confirm the assertion, that the difference of 

 warmth of those parts of the spectrum which closely bor- 

 der upon each other decreases, the more the position of the 

 prism deviates from the normal,which is a result of the greater 

 dispersion of light and the decrease of its intensity. 



Bv 



