40-1 Memoranda respecting Experirneiiis to ascertain 



low as to be below the sun's rays; the dark bhie almost as low; 

 the lighter blue not quite so low as the dark ; the other colours 

 less, as they were lighter ; and the white remained on the sur- 

 face of the snow, not having sunk at all*. 



It is easy to conceive that the next step in their projiress to 

 ascertain the various projjertics of lisrht and heat, would 1)e to 

 examine whether rays of different rcfrangihility give a different 

 degree of heat to those bodies with which they come in contact ; 

 and the best and most rational mode of making this -evident to 

 tlie senses, would be by causing tiie coloured rays the riiSelves to 

 lall imniediutelv on the bulbs of thermometers placed to in- 

 tercept the rays. This was first attempted, I believe, by the 

 Al)be Rochon, in the years 1775 and 177<5- 



The experiments of this gentleman appear to have been passed 

 over with little or no notice, although they form an important link 

 in the jirogress of this department of science. The air thermo- 

 meters which he contrived for showing the effect of the heat on 

 them were extremely ingenious ; yet, it must be owned liis re- 

 sults differ materially from those of later philosopliers: still, how- 

 ever, I think it will be admitted that he is certainly entitled to 

 the honour of h.aving proved, beyond a doubt, that a different 

 degree of heat is given to the thermometer by tlie differently 

 coloured rays of light after they have been decomposed by means 

 of the prism. As he appears to have conducted these experi- 

 ments with the utmost care, and certainly could have had no 

 reason to fahiiy his observations, it is not impossible but that in 

 endeavouring to discover the cause, and to reconcile the ap- 

 parent disagreement between his results and those of others who 

 have followed him, some useful fact may arise, which at present 

 remains imknown. 



The reason why these experiments of his have not been more 

 noticed, may perhaps arise from tlie circumstance of the work 

 not being very commonly to be met with in this country; and as 

 the memoir is but a short one, and may probably be deemed in- 

 teresting by some of your readers, I have translated it, and take 

 the liberty of sending it""you for your valuable Magazine^ should 

 you deem it worthy of being inserted therein f. 



It forms an important link between the brief sketch I have 

 drawer of optical discoveries, and the highly important and later 

 ones of Dr. Herschel, Sir Henry Erfglefield, Dr. Wollaston, Sir 



* Tl>e greater pare of \aiir readers must be acquainted with the very in- 

 genious experiments of Professor Leslie, proving the converse of those re- 

 sults, tliat dark -colo'-«-.d bodies give out their heat much quicker than light- 

 coloured or poiisiied bodies— a fact of groat practical utility, but not ne- 

 cessary to be further dwelt upon in this place. 



t See the nest article, 



Humphry 



