p,i6 On the Vowe-rs of tht pnfmatie Colour's 



thermometer, the other to bring it down to the tempcraHir^ 

 of the room, I fuppoie that, on account ui' the iinalhiefs d 

 tlie ball in Dr. Wilfon's IMo. i, whjch is but little more than 

 4'ti of an inch, the cooling caufes mwft have a flronger effect 

 on the mercury which it contains than they can have on 

 njine, the ball of which is half ah inch. 



More accuracy niay hereafter be obtained by attending to 

 the circumdances of blackfno- the balls of the thermometers, 

 and their expoiure to a more iieady and poweri'ul light of th6 

 fun, at greater altitudes than it can be had at prefent; but 

 the experiments which have been related are quite fufficient 

 for my prefent purpofe; which only goes to prove that the 

 heating power of the prifmatic colours is very far from being 

 equally divided, and that the red rays are chiefly eminent in 

 that refpe6l. 



Experiments on the Uliim'mcitlng Poivcr of coloured Hays. 



In the following examination of the illuniiinating power of 

 diflerently coloured rays, I had two ends in view. The firfl 

 was with regard to the illuniiuaiiofi itfelf, and the next with 

 refpecl to the aptnefs of tlie rays for giving diflindi vifion; 

 and, though there did not fcen.i to be any partictdar reafon 

 why thefe two ihould not go together, I judged it right to at- 

 tend toi both. 



The microfcope offered itfelf as the mod convenient inftru- 

 ment for this invefligationj and I thought it expedient to 

 "view only opaque objeiSs, as thefe would give me an oppor- 

 tunity to ufe a dircft prifmatic ray, without running the; 

 rilk of any bias thSt might be given to it, in its tranfmiflion 

 throueh the colouring particles of tranfparent objefts. 



ijl Experiviait. I placed an obje6t that had very minute 

 parts under a double microfcope; and, having fet a prifm in 

 tlie window, fo as to make the coloured image of the fun 

 ftationary upon the table where the microfcope was placed, I 

 fcaufed the differently coloured rays to fall fuccellively on the 

 bbjeft by advancing the microfcope into their light. The 

 hiagnifving power was 27 times. 



In cha)iging the illumination, bv admitting a different co- 

 \ovw; it always becomes necellarv to re-adjuli the inllrument. 

 It is well known_j that the different rcfrangibility of the rays 



wilJ 



