VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 697 



mometer N° 2, which had been chosen on purpose for a standard, and was within 

 an inch of the other thermometer, remained all the time without the least indica- 

 tion of any change of temperature that might have Min# N o x N o 2> 

 arisen from straggling rays, had there been any such. I o 60 56 



now took away the mirror, but left every thing else in 8 57 5 g 



the situation it was. The effect of this was thus. Here, 10 56 56 



in 10 m , the thermometer N° 1 lost again the 4° it had acquired, while N° 2 still 

 remained unaltered; and this becomes therefore a most decisive experiment, in 

 proof of the existence of invisible rays, of their being subject to the laws of reflec- 

 tion, and of their power of occasioning heat. 



Exper. 8. Reflection and Condensation of the Invisible Solar Rays. — I made 

 an apparatus for placing the small steel mirror at any required angle, fig. 2, pi. 12 ; 

 and having exposed it to the prismatic spectrum, so as to receive it perpendicularly, 



1 caused the colours to fall on one half of the mirror, which, being covered by a semi- 

 circular piece of pasteboard, would stop all visible rays, so that none of them could 

 reach the polished surface. On the pasteboard were drawn several lines, parallel to 

 the diameter, and at the distance of -^ of an inch from each other ; that, by with- 

 drawing the apparatus, I might have it at option to remove the last visible red to 

 any required distance from the reflecting surface. In the focus of the mirror was 

 placed the thermometer N° 2. I covered now also the other half of the mirror, till 

 the thermometer had assumed the temperature of its situation. Then, withdrawing 

 the apparatus out of the visible spectrum, till the last N o 2 



tinge of red was -^ of an inch removed from the edge _ In the Focus of invisible 

 of the pasteboard, and the whole of the coloured image " 6l ' 



thus thrown on the semicircular cover, I opened the other 1 80 



half of the mirror, for the admission of invisible rays. 2" 72 



The result was as annexed. Here, in 1 minute the 4 ^ 4 



thermometer rose 19 degrees. I covered the mirror. 



Here, in 3 minutes, the thermometer fell l6°. I opened In ^ F f c ° us 2, of invisible 



the mirror again. Here, in 2 minutes, the thermo- Min. Heat, 



meter rose 24°. I covered the mirror once more. g g 8 



And, in 1 minute, the thermometer fell 19 . Now by 7 m qq 



this alternate rising and falling of the thermometer, 3 



points are clearly ascertained. The first is, that there are invisible rays of the sun. 

 The 2d, that these rays are not only reflexible, in the manner which has been proved 

 in the foregoing experiment, but that, by the strict laws of reflection, they are 

 capable of being condensed. And, in the 3d place, that by condensation, their 

 heating power is proportionally increased ; for, under the circumstances of the ex- 

 periment, we find that it extended so far as to be able to raise the thermometer, in 



2 minutes, no less than 24°. 



Exper. 9. Reflection of Invisible Culinary Heat. — I planted my little steel mirror 

 on a small board, fig. 5, pi. 12 ; and at a proper distance opposite to it I erected a 

 vol. xviii. 4 U 



