6qo philosophical transactions. [anno 1800. 



I might now have gone on to the 5th line; but, so fine a day, with regard to 

 clearness of sky and perfect calmness, was not to be expected often, at this time of 

 the year; I therefore hastened to make a trial of the other extreme of the prismatic 

 spectrum. This was attended with some difficulty, as the illumination of the violet 

 rays is so feeble, that a precise termination of it cannot 

 be perceived. However, as well as could be judged, I 

 placed the thermometers 1 inch beyond the reach of 

 the violet rays, and found the result as annexed. Here 

 the several indications of the thermometers, 2 of which, 

 N c 1 and 2, were used as variable, while the 3d was 

 kept as the standard, were read off during a time that lasted 12 minutes; but they 

 afford, as may be seen by inspection, no ground for ascribing any of their small 

 changes to other causes than the accidental disturbance which will arise from the 

 motion of the air, in a room where some employment is carried on. 



I now exposed the thermometer to the line of the very first perceptible violet 

 light; but so that N° 1 and 2 might again be in the 

 illumination, while N° 3 remained a standard. The 

 result proved as annexed. Here the thermometer N° 1 

 rose 1° in 15 minutes; and N° 2 rose 4-°, in the same 

 time. From these last experiments, I was now suffici- 

 ently persuaded, that no rays which might fall beyond the violet, could have any 

 perceptible power, either of illuminating or of heating; and that both these powers 

 continued together throughout the prismatic spectrum, and ended where the faintest 

 violet vanishes. 



A very material point remained still to be determined, which was, the situation 

 of the maximum of the heating power. As I knew already that it did not lie on 

 the violet side of the red, I began at the full red colour, and exposed my thermo- 

 meters, arranged on a line, so as to have the ball of N° 1 in the midst of its rays, 

 while the other 2 remained at the side, unaffected by N° l. N°2. N°3. 

 them. Here the thermometer N° 1 rose 7° in 10 mi- *?* 4 **f 48 



oo$ 48§ 48 



nutes, by an exposure to the full red coloured rays. 55J 4s| 48 



I drew back the stand, till the centre of the ball of N° 1 was just at the vanishing 

 of the red colour, so that half of its ball was within, N° l. N° 2. N°3. 

 and half without, the visible rays of the sun. Here Jgl *ff f J 



_ . • 8 4oj 48 



the thermometer N 1 rose 8° in 10 minutes. 57 49 48^ 



By way of not losing time, in order to connect these last observations the better 

 together, I did not bring back the thermometer N° 1 

 to the temperature of the room, being already well 

 acquainted with its rate of showing, compared to that 

 of N° 2, but went on to the next experiment, by 

 withdrawing the stand, till the whole ball of N° 1 was 

 completely out of the sun's visible rays, yet so as to bring the termination of the 



