of the prismatic Colours to heat and illuminate Objects. 273 



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 

 becqme 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 disk 

 of the sun, I prepared 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 

 telescopes : - 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. 



N n 2 



