Ohfervations and Experiments on Light and Heat. J47 



feet ; and though the fhutter was clofed, fufficient light entered by the edges of the prifnx 

 for obferving the inftrument. In two or three minutes, as the colour travelled along the 

 black ball, the fluid generally attained the point of equilibrium. Photometers of both 

 conftru£tions were employed with fimilar refults. The obfervations were frequently re- 

 peated, and from the mean of them all it appears ; that, diftinguifliing the fpectrum into 

 four equal portions, which coincides with the blue, the green, the yellow, and the red 

 the correfponding intenfities or calorific meafures, are i, 4, 9, and 16 degrees, as in fig. 2. 

 PL XV. Hence the gradation of thofe intenfities will be denoted by a parabola referred to 

 its vertical tangent. A variety of curious propofitions may hence be deduced. Thus, fince 

 the complimentary fpace of the parabola is the third part of the circumfcribing re£l:angle, it 

 follows that the intenfity of the red rays feparated by a prifm of flint glafs is triple that of the 

 compound white beam ; and for the fame caufe the blue rays are more than five times 

 feebler than the common ftandard. It is not without reafon that I ftate- the particular fort 

 of prifm ufed, becaufe with one of another fubftance the colours might be expanded after 

 proportions materially different, which would of courfe give different refults j for the force 

 of a pencil of coloured light muft evidently depend on the denfity, as well as on the 

 fpecies of the rays. 



The important difcovery of the achromatic telefcope decifively evinces that refrangibi- 

 lity is not a property inherent eflentially in the particles of light, but refults from the fpecific 

 relation or affinity between thofe various particles and the refringent medium. In what 

 proportions the feveral rays enter into the compoCtion of the folar beam has not been 

 determined, nor does any mode of folving the problem eafily occur. 



It is remarkable that all what is really valuable in Newton's optical difcoveries was an- 

 nounced at an early age, but encountered fuch oppofition or experienced fuch negle£t, as 

 to afFedl that great man with chagrin, and to extinguifh completely the dtfire of communi- 

 cating his views at large to the world. Nor did he refume that taflc till near the clofe of 

 life, when worn down with infirmities, and- his choiceft faculties wafted in the compofi- 

 tion of laborious works, which, for his credit with pofterity, had better been fufFered to 

 reft in oblivion. Yet even thefe (fuch is the efFe£l of exalted reputation I) were now ac- 

 cepted with indifcriminate applaufe. Had his treatife of optics been publifhcd at an ear- 

 lier and happier period, it would certainly have been more fober and correfl:- The fep- 

 tenary and mufical divifion of the prifmatic fpeftrum is devoid of foundation, and too 

 plainly betrays a tindlure of the myfticifm of the age. It is equally ftrange and mortifying 

 to obferve the moft objeftionable part of that fyftem, and which is direftly confuted by 

 the theory of achromatic glafl"es, ftill repeated in popular books, and even admitted by fome 

 authors of a higher clafs. 



While I was occupied with thofe obfervations in Fifelhire, fome vague accounts 

 reached me of a paper communicated to the Philofophical Society at London. And 

 whatever my fentiments were refpe£ling the validity of the conclufions, I refolved calmly 

 and impartially to fubje£t the pretended fafts to the teft of experiment. When a pho- 



Y y 2 tometer' 



