l c 26 0N THE SKPARAtlOW Of LlGrft AND HfiAfJ 



It was fupported by an horizontal arm, projecting from Z 



pole like that of a fire-fcreen. The arm could be adjufted 



to any height by a fcrew ; and the prifm was like wife capable 



of being turned on its axis to any required pofition. 



7m C f P e< ^ rum The coloured fpe&rum was thrown on a very good lens, 

 fell on a lens. . \, \ J ° 



ot tour inches m diameter, and about 22 inches focal length. 



This lens formed part of a machine well known, and ufed 

 for viewing prints; it ftands on a foot adjuftable in any di- 

 rection, and to any height; and the mounting being of wood* 

 and merely fufficient to fupport the lens, fcarcely any heat 

 can be accumulated in any part of it. The whole lens, as 

 well as its mounting, was covered with a thick white pafte- 

 Through a flit board fcreen, in which was cut a flit of 3 inches long, and 



one colour only na ^ an * ncn vv ide; tms ^' l was over tne centre of the lens; 

 to pafs. and through it any one of the colours required was admitted 



on the lens, while all the reft of the fpedlnim was totally ex- 

 cluded. A light wooden arm, of two feet long, projected 

 at right angles from the lower part of the mounting of the 

 lens. This arm carried a fmall fcreen of poliflied card, which 

 received the image formed in the focus of the lens. This was 

 found neceffary, in order to find with certainty where to place 

 the thermometer; when the focal diftance was found, the 

 fcreen was moved back, about a diameter of the bulb of the 

 thermometer ufed, which was then held by the hand in the 

 focus of the lens, which was done with great eafe and cer- 

 tainty, as nothing more was neceflary than to fix the eye on. 

 the card fcreen, and keep the ball of the thermometer in the 

 centre of the luminous image. The whitenefs and polith of 

 the fcreen totally precluded any accumulation of heat in it; 

 nor indeed would fuch accumulation have been of any detri* 

 ment to the experiments had it exifted ; for, as it mult have 

 been entirely owing to the ray under examination, it would 

 have done nothing more than increafe the effect of it on the 

 thermometer. 

 A mercurial T/he thermometers ufed were mercurial, and very fenfible, 



was held in the The fcales were ivory tubes, embracing the ftem, and gra- 

 focus, duated within. The ball of the inftrument was therefore 



unconnected with any mounting ; and no falfe heat could pof* 

 fibly affect it. The balls were moftly blackened with Indian 

 ink, carefully laid on ; but fome were ufed naked, and one, 

 covered with white water-colour paint. 



The 



