An Essay on the Degree of IVarmih of coloured Rays, 41 1 



a thermometer a sufficient quantity of these rays, to produce ef- 

 fects that are evident ; or else to use thermometers of a sensi- 

 bility proportionate to the loss of intensity which is inseparable 

 from the hght thus decomposed. We can always collect into a 

 small space a quantity of rays of a colour that will be sensibly 

 homogeneous, either by disposing in a proper manner several 

 prisms of different angles which shall make each species of rays 

 to coincide on the ball of a thermometer ; or by using the burn- 

 ing mirror of M. de Buffon, known by the name of the Mirror 

 of Archimedes, whose burning line is very nearly the same for 

 a considerable length. 



This instrument is an assemblage of small plane mirrors, which 

 by their respective positions will cause as large a quantity of 

 light as may be desired to fall on a prism placed in their com- 

 mon focus. Now the united rays which form this focus, or 

 rather this burning line, approach so much the nearer to the 

 parallelism necessary for the success of the experiment in ques- 

 tion, as the distance of the prism from the mirrors is more con- 

 siderable. 



We know that this method requires a large room ; but the 

 one in which the experiments I am going to relate were made, 

 not being of a proper size, I thought that very sensible thermo- 

 meters would sufficiently fulfil the object which I proposed. 



Those wliich I have used were air-thermometers. It is well 

 known tliat tliey owe their effect to a bubble of air introduced 

 into the ball vvhicli contains the fluid. The slightest degree of 

 warmth dilates this bubble, and causes the spirit of wine to 

 mount in the capillary tube of the thermometer : but this ascent 

 would scarcely be sensil)le if the top of the tube were hermetically 

 sealed ; it is therefore necessary to leave it open, and conse- 

 quently the exterior air, which is susceptible of many variations, 

 acts on the fluid in such a way that it is necessary to have re- 

 course to the barometer to know the eflFect of the dilatation, 

 which is found to correspond with the greater or less elasticity 

 of the exterior air. 



These defects, joined to the evaporation of the liquor, obliged 

 me to reject these thermometers, notwithstanding their prodi- 

 gious sensibility. AUhough in my later researches 1 have not 

 had these iucooveniences to discourage me, it being sufficient to 

 compare the different effects of each kind of rays that took place 

 in a very short space of time, I have nevertheUss endeavoured** 

 to avoid them alto^jether, bv soldering to tlic t'lbe <.rthe thermo- 

 meter a liirge ball filled with air, which cuitains a common 

 thermometer. This ball is dipped into a .fluid whose tempera- 

 ture i^ (.'iisily kept uniform, whether we choose to have it at the 

 freezing point or at any other temperature. 



Bv 



