2^8 Experiments on the Solar and on the 



focus of the red rays, or a very little beyond it, I had placed 

 the ball of the thermometer No. i ; and, as near to it a3 con- 

 venient, the fmall one No. 2. Now, that the invifible folar 

 rays which occafion heat were accurately refracted to a focus, 

 may be feen by the following account of the thermometers : 

 No. i. No. 2. -\ Here, in one minute, thefe 



In the Focus. Near the Kocus. I ravs ca\ e 45 degrees of heat 

 o' 57 57 j to the thermometer No. i, 



1 103 57 y which received them in the 



focus, while the other, No. 2, differed no change. 



It is remarkable that, notwithstanding I kept the red co- 

 lour of the fpectrum ~ b of an inch upon the pafteboard, a 

 little of that colour might ftill be feen on the ball of the ther- 

 mometer. This occasioned a furmife that, poftibly, the in- 

 vifible rays of the fun might become vifible if they were pro- 

 perly condenfed ; I therefore put this to the trial, as follows : 



I Sth Experiment. Trial to render the invifible Rays of the. 

 ( Sun vijible by Condenfation. 



Leaving the arrangement of my apparatus as in the laft 

 experiment, I withdrew the lens, till the laft vifible red colour 

 was two-tenths of an inch from the margin of the femicir- 

 cular pafteboard cover; then, taking the thermometers, I had 

 as follows : 



No. 2. No. 3.^ Here there was no longer the leaft 

 o' 57 57 f tinge of any colour, or veftige of 



I 78 57 J light, to be feen on the ball of 



the thermometer; fo that, in one minute, it received 21 de- 

 grees of heat, from rays that neither were vifible before, nor 

 could be rendered fo by condenfation. 



To account for the colour which may be feen in the focus, 

 when the laft vifible red colour is lefs than two- tenths of an 

 inch from the margin of the pafteboard which intercepts the 

 prifmatic fpectrum, we may fuppofe lhat the imperfect re- 

 fraction of a burning lens, which, from its great diameter, 

 cannot bring rays to a geometrical focus, will bring fome 

 fcattered ones to it, which ought not to come there. We 

 may alfo admit that the termination of a prifmatic fpectrum 



cannot 



