ON THE SEPARATION OF LIGHT AND HEAT. ] 29 



Yhe apparatus all the fame as in the former. 



In the full red ray. Black therm. 



Black thermometer in 3', from 67 • to 7 1 \ ° J^" d £* 



Quite out of the ray. 



Black thermometer in 3', from 68° to 77|°. 



The ray was now fo far removed from the (lit in the fcreen, Darker io|<\ 

 that fcarce any light was perceptible in the focus of the lens. 

 The black thermometer was now placed near half an inch 

 from the bound of the vifible light in the focus, and rofe in 

 3', from 69° to 79f°. 



The utmoft edge of the prifmatic fpeclrum was now removed Moft dark 9'. 

 an eighth of an inch from the edge of the Hit in the fcreen ; 

 and no light was now vifible in the focus of the lens. 



Black thermometer in 3', from 70° to 79°. 



Mr. Gary, optician in the Strand, and Dr. Hunter, were The difference 

 prefent at thefe experiments, and repeatedly faw the thermo~ P" ceiva le J 

 meter, in the fecond experiment, fink when carried into the 

 lfght,' and rife again when removed back into the dark. Dr. 

 Hunter alfo received the focus on the palm of his hand, where 

 the heat was fenfibly felt; and on fhutting his eyes, and point- 

 ing with a long pen to where the heat was greateft, he always 

 touched his hand beyond the vifible light. 



As the red image has been continually mentioned in the Defcriptlon of 

 courfe of the above recited experiments, it may not be im- t e re ,ina S e ' 

 proper to defcribe it more particularly. The diameter of the 

 red foot, formed by the ray in the focus of the lens, was juft 

 two tenths of an inch in diameter, at right angles to the length 

 of the fpe&rum, and well defined : in the direction of the 

 fpe&rum it was elongated, as might be expected, and lefs 

 well defined. 



When the whole vifible fpeclrum from the prifm was re- Very faint red ia 

 ceived on the fcreen which covered the lens, and the utmoft ?® JS^JJ, 

 edge of the red rays was removed a full eighth of an inch greateft. 

 from the edge of the flit in the fcreen, there was ftill a faint 

 blufh of red, of a femioval form, vifible when the focus of 

 the lens was thrown on a white fcreen ; and it was in thefe 

 circumftances that the greateft effort of heat was conftantly 

 produced on the thermometer^ not by placing it in the red 

 light, but out of it, in the axis of the lens. 



I have only to add, that in the courfe of the month of June Repetition of 

 1802, I repeated moft of thefe experiments with the' fame experiments, 



Vol. Ill, — October, 1802. K apparatus, 



