On the Heat and Illumination afforded by coloured Rays. 32 1 



meter, and of fufficient length to permit the whole of one of the prifmatic colours to pafs 

 through; fo that in the a£tual exhibition, this flit vas placed parallel to the axis of a prifm 

 through which the folar rays pafles, and the furface of the pafteboard was adjulled at right 

 angles to the ray itfelf. Three delicate thermometers with their balls blacked, and fuffi- 

 ciently detached from their refpedtive fcales, were placed on the platform of the frame, at 

 fuch a diftance beneath the opening in the pafteboard, that any one, or all of them, might at 

 pleafure be expofed to the coloured rays, or fhaded from the light by the interpofiuon of the 

 unperforated (urface. When one of thcfe was expofed to the rays, its mercury rofe, while 

 that in the other two remaining ftationary, proved, that the accefljon of heat was pro- 

 duced by the mere a6lion of the folar light. Eight fets of experiments were tabulated, 

 from which it is feen, that the mean rife in one of the thermometers in red rays, during 

 ten minutes was 6|- degrees ; and in the green rays 3J degrees ; and in the violet rays two 

 degrees. With a fmaller thermometer the fame effe£t followed, but the alteration was lefs, 

 probably from the cooling agency of the afcending ftream of air, %vhich would aft more 

 ftrongly where the proportion of furface to bulk was greater. The mean refults from 

 both thermometers were, that the degrees of elevation in the red, green, and violet rays, 

 proved nearly as the numbers 3I-, i^, and i refpeftively. 



The fecond courfe of experiments defcribed in this paper, was made on the illuminating 

 power of coloured rays ; in which the author had two ends in view, the firft with regard to 

 the illumination itfelf, and the fecond with refpedt to the aptnefs of the rays for giving 

 diftindt vifion : properties which, though there did not feem to be any reafon why they 

 ftiould not have the fame meafure, appeared neverthelefs to deferve to be feparately at- 

 tended to. 



The microfcope offered itfelf as the moft convenient inllrument for this inveftigation ; 

 and this was ufed as upon opake objeQs, in order to avoid any effeft that might be ex- 

 pected to arife from tranfmiffion through the parts of coloured tranfparent bodies. 



The regifter of a number of experiments, in which different objefls were viewed in the 

 coloured rays by a magnifying power of 42 times, is given; among thefe a nail is men- 

 tioned as peculiarly fuited for refearches of the prefent kind. It was chofen on account of 

 its folidity and blacknefs, as being moft likely to give an impartial refult of the modifica- 

 tions arifing from an illumination by differently coloured rays ; but on viewing it. Dr. H. 

 was ftruck with the fight of a bright conftellation of thoufands of luminous points, fcat- 

 tered over Its whole extent, as far as the field of the microfcope could take it in. Their 

 light was that of the illuminating colour, but differed confiderably in brightnefs : fome ofr 

 the points being dim and faint, while others were luminous and brilliant. The brighteft 

 of them alfo admitted of a little variation in their colour, or rather in the intenfity of the 

 fame colour ; for in the center of fome of the moft brilliant of thefe lucid appearances, 

 their light had more vivacity, and fecmed to deviate from the illuminating tint towards 

 ■ftrhitencfs, while on and near the circumference it feemed to take a deeper hue. 



Voi. IV.— October 1800. Tt The 



