IS Experiments on the Refrangibility 



Here the feveral indications 

 of the thermometers, two 

 of which, No. 1 and 2, 

 were ufed as variable, 

 while the third was kept 

 as the ftandard, were read 

 off during a time that lafted 12 minutes; but they afford, as 

 may be feen by inflection, no ground for afcribing any of 

 their fmall changes to other caufes than the accidental dif- 

 turbance which will arife from the motion of the air in a 

 room where fome employment is carried on. 



I expofed the thermometer now to the line of the very firft 

 perceptible violet light, but fo that No. 1 and 2 might again 

 be in the illumination, while No. 3 remained a ftandard. 

 The refult proved as follows : 



Here the thermometer No. 1 

 rofe 1 degree in 15 mi- 

 nutes ; and No. 2 rofe 

 i- degree in the fame 

 time. 

 From thefe Iaft experiments, I was now fufRcie^tly per- 

 fuaded, that no rays which might fall bevond the violet, 

 could have any perceptible power either of illuminating or 

 of heating; and that both thefe powers continued together 

 throughout the prifmatic fpectrum, and ended where the 

 fainteft violet van; flics. 



A very material point remained ftill to be determined, 

 which was, the fituation of the maximum of the heating 

 power. 



As I knew already that it did not lie on the violet fide of 

 the red, I began at the full red colour, and expofed my ther- 

 mometers, arranged on a line, fo a? to have the ball of No. 1 

 in the midft of its rays, while the other two remained at the 

 fide, unaffected by them. 



Here the thermometer No. 1 

 rofe 7 degrees in 10 mi- 

 nutes, by an expofuie to 

 the full red coloured rays. 

 I drew back 'he ftand, till the centre of the ball of No. 1, 

 was juft at the vanifliing of the red colour, fo that half its 

 ball was within ; and half without, the vifible rays of the fun. 



No. 



