J59 OA the healing Power of coloured Rays. 



I have already extended this paper beyond the limits I firft propofed : I will not, 

 therefore, fatigue your readers by attempting the refutation of fome obfcure and verbofe 

 arguments advanced by Dr. Herfchcl, which involve a fingular fpecies of logic, and feem 

 repugnant to the fundamental principles of dynamics. The hypothefis of invlfible light is 

 not new in the fcientific world. It was fuggefted by the radiant heat of Scheele and the 

 radiant cold of Pi£tet. It was propofed or adopted by the late very ingenious Dr. Hutton. 

 The fads, indeed, on which it was grounded may be fatisfaftorily explained from known 

 principles. It was not confident with drift metaphyfics -, and to bellow the property of 

 being refle£ted on invifible light, was furely ftretchiug the limits of probability. Yet was 

 the hypothefis, in fome degree, plaufible and alluring. The little improvement of afcrib- 

 ing to it likewife refran^ibility, by rendering the whole abfurd, has diffolved the charm. 

 What is the eye itfelf but a compound prifm ? And is not the expanfion of the optic nerve 

 adapted by its conftltution to receive impreffions through the diaphanous fubftance of the 

 humours and coats, and to convey their appropriate fenfations ? But thofe are only fenfations 

 pf light. Senfacions of heat are confined to no particular clafs of nerves. Refrangibility is 

 therefore correlative with vifion, and " invifible refrangible light" feems a contradicftion of 

 terms. ■ But all metaphyfical confideratlons apart ; if the image of the fun be not encircled 

 with a broad lucid ring, on fixing our eye on that luminary, the fenfation of heat ought not 

 to refid&in the correfponding fpot on the retina, it fliould be more intenfely felt over the 

 furrounding fpace. 



Let me conclude by recommending to your inquifitive readers two works of very 

 fuperior rperit. I mean the Traite d'Optique of Bougner, and the Fhotometria of Lambert. 

 The public has a right to expe£l: that authors have previoufly ftudied the labours of their 

 predeceffors; but it would be charity to believe that fome late writers had not confulted 

 thofe excellent models, which might have prevented much unneceflary repetition, and 

 corrected feveral grofs errors. 



< I am, SIRf 



Your moft obedient fervant, 



JOHN LESLIE. 



IV. 



Account of a Memoir of M. Provst, on feveral interefting Points in Ckemiflry*. 



X HE firft article relates to the converfwn of camphor into oil by repeated diftillations from 

 a bolar earth. This operation, he obferves, was long fince defcribed by Neumann, and 

 he appears to reproach Citizen Lagrange that he has not quoted this author in his Memoir 



* Trandated from the Report made to the firft Clafs of the Phyfical and Mathematical Sciences of tlie In- 

 ftittite of France by Cit. Vauquelin ; GuytOn and Vauqueliu being commilfioners. The report is inferted 

 in the Annales de Chimie, XXXV, 31, 



upon 



