VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 683 



beam, if this different refrangibility did not exist, must inevitably fall uniformly on 

 a space equal to the area of the prism ; and if radiant heat were not refrangible 

 at all, it would fall on an equal space, in the place where the shadow of the prism, 

 when covered, may be seen. But neither of these events taking place, it is evident 

 that radiant heat is subject to the laws of refraction, and also to those of the dif- 

 ferent refrangibility of light. May not this lead us to surmise, that radiant heat 

 consists of particles of light of a certain range of momenta, and which range may 

 extend a little farther, on each side of refrangibility, than that of light ? We have 

 shown, that in a gradual exposure of the thermometer to the rays of the pris- 

 matic spectrum, beginning from the violet, we come to the maximum of light, 

 long before we come to that of heat, which lies at the other extreme. By several 

 experiments, which time will not allow me now to report, it appears that the maxi- 

 mum of illumination has little more than half the heat of the full red rays ; and, 

 from other experiments, I likewise conclude, that the full red falls still short of the 

 maximum of heat ; which perhaps lies even beyond visible refraction. In this 

 case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted 

 the expression, of invisible light ; that is to say, of rays coming from the sun, 

 that have such a momentum as to be unfit for vision. And admitting, as is highly 

 probable, that the organs of sight are only adapted to receive impressions from 

 particles of a certain momentum, it explains why the maximum of illumination 

 should be in the middle of the refrangible rays ; as those which have greater or less 

 momenta, are likely to become equally unfit for impressions of sight. Whereas, 

 in radiant heat, there may be no such limitation to the momentum of its particles. 

 From the powerful effects of a burning lens however we gather the information, 

 that the momentum of terrestrial radiant heat is not likely to exceed that of the sun ; 

 and that consequently the refrangibility of calorific rays cannot extend much 

 beyond that of colourific light. Hence we may also infer, that the invisible heat 

 of red-hot iron, gradually cooled till it ceases to shine, has the momentum of the 

 invisible rays which, in the solar spectrum viewed by day-light, go to the confines 

 of red; and this will afford an easy solution of the reflection of invisible heat by 

 concave mirrors. 



Application of the Result of the foregoing Observations, to the Method of viewing 

 the Sun advantageously, with Telescopes of large Apertures and high magnifying 

 Powers. — Some time before the late transit of Mercury over the sun's disc, I pre- 

 pared my 7-feet telescope, in order to see it to the best advantage. As I wished 

 to keep the whole aperture of the mirror open, I soon cracked every one of the 

 darkening slips of wedged glasses, which are generally used with achromatic tele- 

 scopes : none of them could withstand the accumulated heat in the focus of 

 pencils, where these glasses are generally placed. Being thus left without 

 resource, I made use of red glasses ; but was by no means satisfied with their 

 performance. My not being prepared, as it happened, was of no consequence; the 

 weather proving totally unfavourable for viewing the sun at the time of the transit. 



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