508 Dr. Herschel's Experiments on the solar, and 



here, on the contrary, to suppose that the same mechanism 

 should be the cause of such different sensations, as the delicate 

 perceptions of vision, and the very grossest of all affections, 

 which are common to the coarsest parts of our bodies, when 

 exposed to heat ? 



But, let us see what light may now be obtained from the 

 several articles that have been discussed in this Paper. It has 

 been shewn, that the effect of heat and of illumination may be 

 represented by the two united spectra, which we have given.* 

 Now, when these are compared, it appears that those who 

 would have the rays of heat also to do the office of light, must 

 be obliged to maintain the following arbitrary and revolting 

 positions; namely, that a set of rays conveying heat, should 

 all at once, in a certain part of the spectrum, begin to give a 

 small degree of light; that this newly acquired power of illu- 

 mination should increase, while the power of heating is on the 

 decline; that when the illuminating principle is come to a 

 maximum, it should, in its turn, also decline very rapidly, and 

 vanish at the same time with the power of heating. How can 

 effects that are so opposite be ascribed to the same cause ? first 

 of all, heat without light; next to this, decreasing heat, but 

 increasing light ; then again, decreasing heat and decreasing 

 light. What modification can we suppose to be superadded to 

 the heat-making power, that will produce such inconsistent 

 results ? 



We must not omit to mention another difference between 

 light and heat, which may be gathered from the same article 

 of the refrangibility of heat-making rays. It is, that though 

 light and heat are both refrangible, the ratio of the sines of 



• See page 439, and Plate XX. 



