VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 699 



for some proof of the assertion, that rays which occasion heat can ever become 

 visible. But as we shall have an opportunity to say more of this hereafter, I pro- 

 ceed now to investigate the refraction of heat-making rays. 



Exper. 1 1. Refraction of Solar Heat. — With a new 10-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 4 lenses, I caused 

 them to fall on the ball of the thermometer N° 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 in- 

 tersecting courses, without jostling together, and each parcel arriving at last safely 

 to its destined place. As soon as the rays were brought to the thermometer, it rose 

 almost instantly from 6o° 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 8 regular successive refractions ; so that their being subject to 

 its laws cannot be doubted. 



Exper. 12. Refraction of the Heat of a Candle. — I placed a lens of about 1.4 inch 

 focus, and 1.1 diameter, mounted on a small support, at a distance of 2.8 inches 

 from a candle, fig. 6, and the thermometer N° 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- 

 port of the lens had an eccentric pivot, on which it might be turned away from its 

 place, and returned to the same situation again, at pleasure. This arrangement 

 being made, the thermometer was for a few moments exposed 

 to the rays of the candle, till it had assumed the temperature 

 of its situation. Then the lens was turned on its pivot, so 

 as to intercept the direct rays, which passed through the opening 

 in the pasteboard screen, and to refract them to the focus, in 

 which the thermometer was situated. Here, in 3 ra , the thermo- 

 meter received 2-l degrees of heat, by the refraction of the lens. 

 The lens was now turned away. Here, in 3 m , the thermometer 

 lost 2-f degrees of heat. The lens was now returned to its situa- 

 tion. And, in 3 m , the thermometer regained the 2-f degrees of 

 heat. A greater effect may be obtained by a different arrange- 

 ment of the distances. Thus, if the lens be placed at 34- inches 

 from a wax-candle, and the thermometer situated, as before, in the secondary focus, 

 we shall be able to draw from 5 to 8 degrees of heat, according to the burning of 

 the candle, and the accuracy of the adjustment of the thermometer to the focus. 

 The experiment we have related shows evidently, that rays invested with a power of 

 heating bodies, issue from a candle, and are subject to laws of refraction, nearly the 

 same with those respecting light. 



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