1815.] of the Effect of Ice on a Thermometer. 343 



something, though much less j and therefore the small compensa- 

 tion was again reduced hy tiie refrigerating mixture. 



The comparing of the reflection of heat to that of light (though 

 they are in many respects analogous) is apt to lead into some error, 

 because in the latter case the reflector is presented in a particular 

 direction towards a particular source (as to the sun, or to an aper- 

 ture admitting tlie rays) ; but tlie rays of heat are received equally 

 from every point of the surrounding sphere : but if we bear this 

 difference in mind, the analogy between them will afford an evident 

 illustration of the theory. Vision is merely the sensation of rays of 

 light on the retina of the eye. We see a white or coloured object 

 by means only of light reflected by it. Black, we say, reflects no 

 light ; yet we see a black object. If the blackness of the surface 

 were really perfect, and reflected no light, we could not (strictly 

 speaking) see it at all ; we perceive or distinguish it by its interception 

 of other forms and objects ; but we can also magnify it by dioptric 

 lenses or mirrors. To jiut the comparison in a still stronger view, 

 suppose a white object (as a paper box blackened within), and a 

 small hole cut in one of its surfaces, we say we see that hole. Do 

 we see darkness ? Through a convex lens, or by a concave mirror, 

 or by the intervention of two mirrors (placed exactly as in Pictet's 

 experiment, the eye being in the pkice of the thermometer), we 

 may magnify that dark hole. Do we say darkness radiates ? And 

 where is the difference between the two cases ; the one, where the 

 ice in the focus of the second mirror lowers a thermometer in that 

 of the first ; the other, where a dark spot in a similar situation 

 gives tiie sensation of a magnified dark spot on the retina of the 

 eye ? A j)erfect analogy holds here, and applies to all the cases, 

 and removes at once (as it appears to me) all the objections made to 

 Mr. Prevost's theory. It is true that in the quotations I have met 

 with of Mr. Prevost's explanation he has not expressly described the 

 intercept ioji by means of the ice, of radiation aliunde ; but I think 

 it follows by necessary inference that he had it in his mind as a part 

 of his theory, and meant to be so understood. I have said before 

 that his own words I have not met with. 



I have now to answer the suj)poscd objection arising from tlic 

 experiment with the metallic tube ; but If the explanations above 

 given of the several phenomena are satisfactory, 1 have only to 

 trace the analogy between them and this new one. 



1 think we are in possession of facts that warrant the following 

 conclusions : — 



1. That all bodies receiving and admitting heat by their surfaces 

 emit by radiation an equal quantity ; and that as much of what falls 

 on them as they cannot so admit, they reflect. 



2. 'I'hat when the nidiatiori of any Ijody is not compensated by 

 counter radiation, its temperature must decrease. 



.\. That the j)roximity <»r the form either of radiating or reflecting 

 surfaces produces no difference in the quantity or intensity of the 

 rnys received by any given point, provided there is no active cause 



