has a calorific Focus, &?c. 423 



as in the former experiment, it rose from 50 to 54, or gained 

 four degrees. 



Exp. '6. — Having again reduced my thermometer to 50', I 

 placed it on the inclined plane ; and having allowed the violet 

 rays to fail on it, the rise was from 60 to 5 I, or one degree. 



So far these experiments, assimilated as much as occasion 

 required to Dr. Herschel's, would seem to conhrm the inferences 

 drawn hy that celebrated astronomer. However, so early as the 

 year 1802, when at Edinhnrgh College attending Dr. Hope's 

 interesting and scientific lectures on chemistrv, I was led to 

 douht the conclusions drawn from these experiments, but at that 

 early period did not' venture to opj»ose such high authority. 



Exp. 4. — Having finished the former experiments, I removed 

 the pasteboard screen, and in its place interposed a sheet of vvi)ite 

 paper on which the spectrum was received ; it was gi adually 

 approximated to within one inch of the prism, where it was pro- 

 perly fastened on a stand. The spectrum at this distance was 

 one-fourth of an inch in breadth, bounded on the one side by 

 orange and yellow rays, and on the other by blue and violet, 

 while light occupied the middle ; this white light was much more 

 powerful and luminous than the surrounding, sun-beams. In- 

 deed, it appeared exactly like the light concentrated in the focus 

 of a burning-glass, which made me strongly fiur.pect that it was 

 likewise more calorific. Nor was I mistaken ; for, en placing the 

 thermometer in those rays, it rose in ten minutes to 8\^ ; the 

 heat of the neighbouring sun-shine being only 70'. The ther- 

 mometer rose and fell alternately as it was removed in and out of 

 the spectrum. From this experiment we must infer that Dr. 

 Herschel made his experiments not only on the heat of the solar 

 ray, but likewise on the heat accumulated by the converging 

 })ower of the prism. Indeed, it appears rather singular that the 

 action of this well-known instrument should heretofore have been 

 so superficially investigated. Every glass wedge must produce a 

 focal heat, for those rays in the vicinity of the angle must suffer 

 a more ))owerful refraction than tlie others ; consequently at a 

 given distance the heat and light must be conrlensed. Let us 

 .suppose the vertical section of a triangular glass wedge or prism, 

 resting on its base, to be represented in the following diagram : 

 AB, two rays of light, 

 coming from the sun S, 

 ^rrike on the prism j.j 

 '"'DE. The ray B near 

 •iie base is but slightly 

 refracted, whilst on^ 

 the other hand the 

 ray A coming through the upper refracting angle is considerably' 

 D d 4 bent 



