128 Experiments on the Solar and on the 



yth Experiment. Reflection of invijible Solar Heal, 

 On a board of about 4 feet 6 inches long, I placed at one 

 end a fmall plain mirror, and at the other two thermome- 

 ters*. The diftance of No. r, from the face of the mirror, 

 was 3 feet 9^ inches; and No. 2 was put at the fide of it, 

 facing the fame way, but out of the reach of the rays that 

 were to be reflected by the mirror. The colours of the prifm 

 were thrown on a fheet of paper, having parallel lines drawn 

 upon it, at half an inch from each other. The mirror was 

 fiationed upon the paper; and was adjufted in fuch a man- 

 ner as to prefent its polifhed furface, in an angle of 45 de- 

 grees, to the incident coloured rays, by which means they 

 would be reflected towards the ball of the thermometer No. 1. 

 In this arrangement, the whole appaiatus might be with- 

 drawn from the colours to any required diftance, by attend- 

 in°- to the laft vifible red colour, as it (bowed itfelf on the 

 lines of the paper. When the thermometers were properly 

 fettled to the temperature of their fituation, during which 

 time the mirror had been covered, the apparatus was drawn 

 o-ently away from the colours, fo far as to caufe the mirror, 

 which was now open, to receive only the invifible rays of 

 heat which lie beyond the confines of red. The refult was 

 as follows : 



Here, in ten minutes, the ther- 

 mometer No. 1 received four 

 degrees of heat, reflected to it, 

 in the ftricteft optical manner, 

 bv the plain mirror of a New- 

 tonian telefcope. The great 



No. 1. No. 2. 



o' 56 5 6 



57 56 



- 59 5 6 



7 60 56 



1.0 60 56 



regularity with which thefe invifible rays obeyed the law of 

 reflection was fuch, that Dr. Wilfon's fenfible thermometer, 

 No. 2, which had been chofen on purpofe for a (tandard, 

 and was within an inch of the other thermometer, remained 

 all the time without the lead indication of any change of 

 temperature that might have arifen from (haggling rays, had 

 there been any fuch. I now took away the mirror, but left 

 every thing elfe in the fituation it was. The effect of this 



was thus : 



• See Plate IV. fig. 1. 



No. i. 



