308 Dr. Herschei/s Experiments on the solar, and 



which occasion heat can ever become visible. But, as we shall 

 have an opportunity to say more of this hereafter, I proceed 

 now to investigate the refraction of heat-making rays. 



nth Experiment. Refraction of solar Heat. 



With a new ten-feet Newtonian telescope, the mirror of 

 which is 24 inches in diameter of polished surface, I received 

 the rays of the sun ; and, making them pass through a day- 

 piece with four lenses, I caused them to fall on the ball of the 

 thermometer No. 3, placed in their focus. Those who are 

 acquainted with the lines in which the principal rays and pencils 

 move through a set of glasses, will easily conceive how artfully, 

 in our present instance, heat was sent from one place to another. 

 Heat crossing heat, through many intersecting courses, without 

 jostling together, and each parcel arriving at last safely to its 

 destined place. As soon as the rays were brought to the ther- 

 mometer, it rose almost instantly from 60 degrees to 130 ; and, 

 being afraid of cracking the glasses, I turned away the telescope. 

 Here the rays, which occasioned no less than 70 degrees of 

 heat, had undergone eight regular successive refractions; so that 

 their being subject to its laws cannot be doubted. 



12th Experiment. Refraction of the Heat of a Candle. 



I placed a lens of about 1,4, inch focus, and 1,1 diameter, 

 mounted upon a small support, at a distance of 2,8 inches from 

 a candle;* and the thermometer No. 2, behind the lens, at an 

 equal distance of about 2,8 inches ; but which ought to be very 

 carefully adjusted to the secondary focus of the candle. Not far 

 from the lens, towards the candle, was a pasteboard screen, 

 with an aperture of nearly the same size as the lens. The sup- 



• Sje Piate XIII. Fig. z. 



