412 j^n Essay on the Degree of Warmth of coloured Rays. 



By this method the full degree of sensibility is given to th« 

 thermometer, without its having any communication with the 

 external air. 



For experiments of so delicate a nature we know that a per- 

 fectly serene sky is necessary, and an unvarying temperature 

 without the leabt agitation of tlie air : this necessarily reduces 

 the experiments on which we may depend to a very small nuni- 

 bor; and among the great variety of those 1 have made, I ac- 

 knowledge that there is not so much as one v.ith which I am 

 completely satisfied. 



I used a large prism of flint glass, of which the refringent.angle 

 was 45^; I permitted an equal quantity of rays to pass through 

 in each case, and tliev successively traversed the same lens, whicli 

 I inclined according to the sun's rdtitudc ; for I had at that time 

 no heliostata, and the want of this instrument has rendered my 

 results still less accurate. 



All mv experiments were made between the hours' of eleven in 

 the morning and two in the afternoon; mv airrthermometer was 

 too sensible to allow of my approaching it without occasioning 

 variations, so that I observed the divisions by a small telescope 

 at the distance of fifteen feet, and I raised or lowered the in- 

 dexes by means of a string passing over some pulleys. 



Although from tlie year 1775 I have made a great number 

 of experiments, I do not think them worthy of being related, 

 because I was not then sufficiently acquainted with the precau- 

 tions that are necessary in such delicate inquiries : those made 

 in 177'> are more conclusive; they give the ratio of the warmth 

 of the red rays to the violet rays as 8 to 1; for, by v^arming the 

 ball of the thermometer for two minutes with tb.oe two kinds of 

 rays, the index would mark from four divisions and a half to se- 

 ven, for the violet rays ; whilst for the red rays it was necessary 

 to place the index from 39 to 44 divisions. These ohser ations 

 were made in the month of June, Reaumur's thermonjeter only 

 varying during the v.hole time they were made from 16° to 17". 



The following are the observations of this month: 



Violet 4i Red 4* 



6" 40 



' 4i 43 



5| 39 



7 44 



\t is necessary to remark, that it is the red approaching to the 

 orange, andtheviokt approaching to the blu?, that 1 have used. 

 It is so difficult to decompose the light uith sutficient perlcction 

 to obtain the same shade of colour constantly, that we must not 



* Most prohabl)' tliis is an error of tlic press, and ougtit to be 40 It is 

 S' considered in the liCTicral result siiven in psure-JOO. 



be 



