702 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



laws of refraction, nearly equal to those which affect light; and that these rays are 

 invested with a power of causing heat in bodies. 



Exper. l6. Refraction of Fire-heat, by an Instrument resembling a Telescope. It 



occurred to me, that I might use a concave mirror, to condense the heat of the 

 fire in the grate of my chimney, and, reflecting it sideways by a plain mirror, I 

 might afterwards bring it to a secondary focus by a double convex lens ; and that, by 

 this construction, I should have an instrument much like a Newtonian telescope, 

 fig. 10. The thermometer would figuratively become the observer of heat, by 

 being applied to the place where, in the real telescope of the same construction, 

 the eye is situated to receive light. Having put together the different parts, in 

 such a way as I supposed would answer the end, I tried the effect by a candle, in 

 order to ascertain the proper distance of the object-mirror from the bars of the 

 chimney-grate. The front of the apparatus was guarded by an iron plate, with a 

 thick lining of wood; and the 2 thermometers which I used, were parted from 

 the mirrors and lens by a partition, which screened them from the heat that was 

 to be admitted through a proper opening in the front plate, to come at the object- 

 mirror. In the partition was likewise an opening, of a sufficient diameter to 

 permit the rays to come from the eye-glass to their focus, on the ball of the ther- 

 mometer N° 1 ; while N° 4 was placed 

 by the side of it, at less than half an 

 inch distance. In the experiment, the mSo"^^ 

 object-mirror was alternately covered by Covered 

 a piece of pasteboard, and opened again. c ^g red 

 The thermometers were read off every Open 

 minute; but, to shorten my account, I vere 

 only give the last minute of every change. Here, in the first 8 m , the thermome- 

 ter exposed to the effects of the fire-instrument, gained 2° of heat more than the 

 other. In the next 8 minutes, the mirror being covered, it gained 1° less than 

 the other. The mirror being now opened again, it gained, in 5 m , 2% degrees 

 more than the other. When covered 6 m , it gained l-f degree less than N° 4. In 

 the next 10 m , when open, it gained ± degree more; and, in the last 10 m , when 

 the fire began to fail, and the mirror was again covered, it lost 1° more than the 

 other thermometer. All which can only be accounted for by the heat which came 

 to the thermometer through the fire-instrument; and as this experiment confirms 

 what has been said before of the refraction of culinary heat, so it also adds to 

 what has already been proved of its reflection. For, in this fire-instrument, 

 the rays which occasion heat could undergo no less than 2 reflections and 2 re- 

 fractions. 



Exper. 17. Refraction of the Invisible Rays of Solar Heat. — I covered half 

 of Mr. Dollond's burning lens with pasteboard, and threw the prismatic spectrum 

 on that cover, fig. 7 ; then, keeping the last visible red colour -iV °f an inc " "* om 



