34^ Obfervations and Experiment's on Light and Heat, 



the fmalleft quantities of light. Hence the explanation of feveral fa£i:s which occurred in 

 the application of the photometer. Expofed to the fun-beams only, it indicated about one 

 hundred degrees at noon during the fummer months in our climate'; flieltered from thofe 

 dire£l rays it gives feventy or eighty degrees, when the Iky is covered with fleecy cloudsi, 

 but only ten or fifteen degrees if the atmofphere is clear and of azure hue. On the fummits 

 of elevated mountains, the fun would be more refplendent while the vault of heaven, of 

 the deepeft blue, would emit a proportional feeble light. The red glofs of a fire was found to 

 make at leaft three or four times as much impreffion as the flame of a candle, under the 

 vifual angle. And language, that grand monument of human thought and feeling, teftifies 

 the conformity of thofe remarks with common fentiment. Red and orange are termed •warm 

 tints, and we admire the glow of the Italian landfcape. Among the colours fcarlet is rec- 

 koned glaring, green, foft, and blue, faint. The pleafurable fenfation excited by the fight 

 of green, independent of its aflbciation with verdure and vegetable life, has been referred 

 to the circumftances of its occupying the middle rank of the fyfliem of colours. But green 

 has more affinity to the dark and abforbent fliades, and the eye feems more gratified with 

 deficiency than excefs of light. 



On a hafty trial, I perceived that, the coloured fpaces of the prifmatic fpe£trum manl- 

 feftcd analogous properties; but as the quantities were fmall and required nicer obfcrva- 

 tion, and as the feafon was already far advanced (it was in Oftober 1797), I refolved to 

 poftpone the whole till the next fummer. I prepared accordingly to refume the invefl:iga- 

 tions with every advantage ; but I am forry to fay that my plan was not accompliflied fo 

 foon as I expected, owing partly to our variable fky, fo difcouraging for optical experi- 

 ments, and partly to various unforefeen interruptions, efpecially that of undertaking a tour 

 through the north of Europe. In the beginning of lad fummer, however, my wifti was 

 realized, yet not without a tedious wafl;e of time ; for fuch is the nature of this infular 

 climate, that the (ky at mid-day is feldom cloudlefs, and, even then, its azure is generally 

 adulterated by a milky haze. I cut a broad horizontal flit in the window-lhutter, fuflicient 

 to admit freely an excellent prifm of flint glafs, which was fixed with wedges at the ends 

 in a poGtion to produce the greateft refraftion. Behind was placed a ftand about i} inches 

 broad, ^ of an inch thick, and of the height of 5 or 6 feet, to which an arm projedling 

 2 or 3 inches from the edge was adapted to Aide upwards or downwards at pleafure, 

 and to this the photometer was attached ; and, to avoid as much as poflible every chance of 

 deception, I annexed to that arm a flender branch fl;retching three or four inches towards 

 the window, and bearing a fmall fcreen capable of being raifed -or depreflTed as occafion re- 

 quired, fo as to intercept at a difl;ance the whole of the coloured rays but thofe fubmitted 

 to examination. 



I {hall omit mentioning other precautions ufed; to thofe who arc acqainted with accurate 

 experimenting it would be fuperfluous, and to the reft of your readers it would appear dif- 

 guftingly minute. The ftand was approached as near to the window as was compatible with 

 the developement of the feveral coloured fpaces, which was at the diftance of about two 



feet; 



