LIGHT. 105 



as to the emittent body. That which is, or becomes, light 

 when it falls upon one body is not light when it falls upon 

 another. Probably the retinae of the eyes of different persons 

 differ to some extent in a similar manner ; and the same 

 substance, illuminated by the same spectrum, may present 

 different appearances to different persons, the spectrum 

 appearing more elongated to the one than to the other, so 

 that what is light to the one is darkness to the other. A 

 dependence on the recipient body may also, to a great extent, 

 be predicated of heat. Let two vessels of water, the contents 

 of the one clear and transparent, of the other tinged by some 

 colouring matter, be suspended in a summer's sun ; in a very 

 short time a notable difference of temperature will be observed, 

 the coloured having become much hotter than the clear liquid. 

 If the first vessel be placed at a considerable distance from 

 the surface of the earth, and the second near the surface, the 

 difference is still more considerable. Carrying on this ex- 

 periment, and suspending the first over the top of a high 

 mountain, and the second in a valley, we may obtain so great 

 a difference of temperature, that animals whose organisation 

 is suited for the one temperature could not live in the other, 

 and yet both are exposed to the same luminous rays at the 

 same time, and substantially at the same distance from the 

 emittent body the substance nearer the sun is, in fact, colder 

 than the more remote. So, with regard to the medium trans- 

 mitting the influence : a greenhouse may have its tempe- 

 rature, or the vegetable growth within it, considerably varied 

 by changing the glass of which its roof is made. 



These effects have an important bearing on certain cos- 

 mical questions which have lately been much discussed, and 

 should induce the greatest caution in forming opinions on 

 such subjects as light and heat on the sun's surface, the tem- 

 perature of the planets, &c. The latter may depend as much 

 upon their physical constitution as upon their distance from 

 the sun. Indeed, the planet Mars gives us a highly probable 

 argument for this ; for, notwithstanding that it is half as far 

 again from -the sun as the earth is, the increase of the white 

 tracts at its poles during its winter, and their diminution 

 during its summer, show that the temperature of the surface 



