On the Heat afforded by coloured Rayt, 345 



of experiment, efpecially if they difregard analogy and the general concurrence of efta- 

 bliflied fadls. It would appear that this able aftronomer, on entering into a new line of 

 enquiry has neither employed apparatus fuited to the nicety of the fubjedl, nor guarded 

 fufficiently againft the numerous and latent fources of erron I confider myfelf entitled to 

 fpeak with the greater confidence, becaufe I had long directed my refearches in the fame 

 channel, and had fortunately hit upon a fimple contrivance, improved fince by perfevering 

 attention which afforded to fuch inveftigations a degree of precifion and facility not hitherta 

 attained. I do not hefitate, therefore, to maintain that Dr. Herfchel's capital propoCtion 

 originates in fallacious obfervations. To fupport this affcrtion I will briefly and with re- 

 ludlance ftate the refult of my enquiries, and will then offer fome remarks or ftriftures on 

 the article in queftion. 



In defcribing my photometer I noticed, in a curfory manner, the feveral points in optics and 

 chemical philofophy for the examination of which it was, by its peculiar delicacy, fo happily 

 calculated. It was not then my defign to enter into details, or relate the fuccefs of the 

 experiments. Thofe interefling deduiSlions I referved for a work which has been feveral 

 years in contemplation, but of which I have deferred the publication from time to time as 

 the materials accumulated, from a defire ftill to extend and correal: my views, and perhaps 

 from a growing indifference to the voice of public opinion, which I am forry to obferve is, 

 in this country at lead, notorioufly fervilc and undifcerning. Nor will I at prefent antici- 

 pate more than what immediately concerns the fubject under difcuffion. 



After conftrudling the inftrument one of my firft objects was to determine, by their calo- 

 rific effefts, the relative intenfities of different colours. The problem had indeed been at- 

 tempted before, by Franklin, Watfon, and others 5 but from fuch rude trials nothing in any 

 degree accurate could be expelled. I therefore coated the abforbent ball of a photometer 

 fucceffively with a feries of water-colours, and, after allowing it to dry, I placed it with ano- 

 ther photometer in the fame fteady light, whether expofed to the diffufive influence of the 

 fky, or to the direft impulfion of the folar beams. The ratio of the quantities marked by 

 the two inftruments gave the proportional abforption of the coloured fubftance, and confe- 

 quently its power of refle£tIon. Inflead of paint, I fometimes covered the ball with bits 

 of coloured filk ; and when two colours fliowcd nearly equal effe£ts, I had recourfe to an 

 expedient which rendered their difference very fenfible. I made the balls of the photome- 

 ter larger than ufual, brought them to the fame elevation, and bent them back in oppofitc 

 direftions ; the glafs-cafe confided of the larger fegment of a hollow fphere furmounting 

 a cylinder ; the zero of the fcale was placed about the middle of one of the branches, and 

 the divifions were marked both in afcent and defcent : on applying the approximate colours 

 to thofe countera£ling balls the difference, whether in excefs or defect, of their abforbent 

 powers was eafily determined. Thofe experiments were likewife varied by reflecting the 

 rays of the fun, at a given angle from a coloured furface, againft the dark ball of a photo- 

 meter. I need not defccnd to particulars, but I was ftruck with the copious reflection 

 from fcarlet, little inferior to that from a white furface. Blue, and next to it green, reflefted 



Vol. IV. — November 1800. Y y th« 



