302 Dr. Herschel's Experiments on the Solar, and 



place 2-f to the account of reflection. The apparatus becoming 

 now very hot, it would not have been fair to have continued 

 the experiment for a longer time ; but the effect already pro- 

 duced was fully sufficient to shew, that even a prism, which 

 stops a great many heat- making rays, still reflects enough of 

 them to prove, that an open fire not only sends them out, but 

 that they are subject to every law of reflection. 



ytb Experiment. Reflection of invisible Solar Heat. 



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

 a small plain mirror, and at the other, two thermometers*. The 

 distance of No 1 , from the face of the mirror, was 3 feet 9^ 

 inches ; and No. 2 was put at the side of it, facing the same 

 way, but out of the reach of the rays that were to be reflected 

 by the mirror. The colours of the prism were thrown on a 

 sheet of paper having parallel lines drawn upon it, at half an 

 inch from each other. The mirror was stationed upon the paper; 

 and was adjusted in such a manner as to present its polished 

 surface, in an angle of 45 degrees, 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 appa- 

 ratus might be withdrawn from the colours to any required 

 distance, by attending to the last visible red colour, as it shewed 

 itself on the lines of the paper. When the thermometers were 

 properly settled to the temperature of their situation, during 

 which time the mirror had been covered, the apparatus was 

 drawn gently away from the colours, so far as to cause the 

 mirror, which was now open, to receive only the invisible rays 

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

 as follows. 



* See Plate XII. Fig. 4. 



