Ig. Experiments on tie Ref Tangibility of 



inch /rom the edge. Thefe lines I interfered at right angle* 

 with three others ; the fecond and third whereof were^ re- 

 fpectivelv, at 2'- and at 4 inches from the firft. 



The fame thermometers that have before been marked 

 No. 1, 2, and 3, mounted upon their fmall inclined planes, 

 were then placed fo as to have the centres of the fhadow of 

 their balls thrown on the intersection of thefe lines. Now, 

 fetting my little Hand upon a table, I caufed the prifmatic 

 fpeclrum to fall with its extreme colour upon the edge ot 

 the paper, fo that none might advance beyond the firft line. 

 In this arrangement, all the fpectrum, except the vanifhing 

 lad quarter of an inch, which ferved as a direction, palled 

 down by the edge of the ftand, and could not interfere with 

 the experiments. I had alfo now ufed the precaution of 

 darkening the window in which the prifm was placed, by 

 fixing up a thick dark green curtain, to keep out as much 

 light as convenient. 



The thermometers being perfectly fettled at the tempera- 

 ture of the room, 1 placed the ftand fo that part of the red 

 colour, refracted by the prifm, fell on the edge of the paper, 

 before the thermometer No. 1, and about half way, or !-£ 

 inch, towards the fecond : it confequently did not come be- 

 fore that, or the third thermometer, both which were to be 

 my ftuudards. During the experiment, I kept the laft ter- 

 mination of vifible red carefully to the firft line, as a limit 

 affigned to it, by gently moving the ftand when required ; 

 and found the thermometers, which were all placed on the 

 fecond line, affected as follows : 



Here the thermometer No. I 

 role 6! degrees in 10 mi- 

 > nutes, when its centre was 

 placed ^ inch beyond vi- 

 fible light. 

 In order to have a confirmation of this fact, 1 cooled the 

 thermometer No. 1, and placed No. 2 in the room of it: 

 I alio put No. 3 in the place of No. 2, and No. 1 in that of 

 No. 3 ; and, having expofed them as before, arranged on the 

 fecond line, I had the following refult: 



No, 



