404 Mr. Powell on Solar Light and Heat. [June, 



other effects) light must produce a heating effect on the black 

 surface of the retina. 



Every different shade of colon?- and every different intensity of 

 light transmitted from objects, produces a different degree of 

 heat on a black surface. It, therefore, follows, that when 

 objects are painted on the retina, the rays coming from different 

 parts of them communicate different degrees of heat by the 

 absorption of its black coat, according to their different colour, 

 brilliancy, &o. And since all our distinction of objects by the 

 eye depends on their colour and reflecting power — on the differ- 

 ently coloured rays, and the total intensity of rays which they 

 reflect, it is at least certain that the perception of objects, and 

 their different parts, must be accompanied by corresponding per- 

 ceptions of a difference of heating effect, whatever other distinc- 

 tive impressions the different rays may be capable of producing 

 on the sentient substance. I am far from meaning to assert the 

 opinion that this is the immediate cause of vision. These obser- 

 vations are merely proposed as affording a curious topic of phy- 

 siological inquiry. 



The heating effect of light is produced at the moment of 

 absorption, and is probably of a different nature from the com- 

 munication of heat either by contact or by radiation. If, there- 

 fore, a non-luminous body could radiate different degrees of heat 

 from its different parts so as to impinge on the retina, it would 

 not produce the phenomenon of vision; but this state of circum- 

 stances cannot take place, since the transparent parts of the eye 

 are not permeable to simple radiant heat ; and if they became 

 heated themselves, there would not be any distinction of different 

 degrees of heat communicated to different parts of the retina. 

 The optic nerve is the only one in the body expanded with an 

 absorbing surface so as to be exposed to the external influence 

 of radiant heating agents. 



(23.) The consideration of the heating effects of light to the 

 phenomena of vision must be attended to in the attempt to 

 compare the illuminating with the heating power of light. 



From the experiments of Sir W. Herschel, it appears that the 

 greatest illuminating effect belongs to the yellowish-green rays. 

 According to his theory, the radiant heat is separate from the 

 light, and is formed of particles having less momentum, those 

 of the green rays having such a momentum as is best suited to the 

 eye for the purposes of vision, either too little or too great a 

 momentum (as in the violet and red rays) being equally ill 

 adapted for producing a vivid impression on the sight. 



Independently of all hypothesis, the above remarks are of a 

 nature deserving great attention in the consideration of these 

 phenomena. The illuminating power of light must be referred 

 not merely to any inherent properties of the rays, but to the 

 nature of our organs. Hence, according to an observation just 

 made, it must be inferred that either too little or too great an 



