290 Dr. Herschel's Experiments on the Refrangibility 



order to obtain a true maximum, as the sun was now more 



powerful than it had been at that time; for which reason, I 



caused the line of termination of visible light, now to fall again 



just \ inch from the centre of the ball ; and had the following 



result. 



No. 1. No. 2. No. 3. 



5°t 5°i - - 50 



57i - - 50 49j 



5H 50 49t 



5H - - 50 49i 



And here the thermometer No. 1 rose, in 16 minutes, 8 J 



degrees, when its centre was -| inch out of the visible rays of 



the sun. Now, as before we had a rising of 9 degrees, and 



here 8|, the difference is almost too trifling to suppose, that 



this latter situation of the thermometer was much beyond the 



maximum of the heating power ; while, at the same time, the 



experiment sufficiently indicates, that the place inquired after 



need not be looked for at a greater distance. 



It will now be easy to draw the result of these observations 



into a very narrow compass. 



The first four experiments prove, that there are rays coming 



from the sun, which are less refrangible than any of those that 



affect the sight. They are invested with a high power of heating 



bodies, but with none of illuminating objects ; and this explains 



the reason why they have hitherto escaped unnoticed. 



My present intention is, not to assign the angle of the least 



refrangibility belonging to these rays, for which purpose more 



accurate, repeated, and extended experiments are required. But, 



at the distance of 52 inches from the prism, there was still a 



considerable heating power exerted by our invisible rays, one 



