206 SENSE OF SIGHT. 



give a succinct view, not only of the eye, properly so called, but also 

 of these accessory organs, which serve to lodge, move, protect, and 

 lubricate it. The description will not, however, be clearly understood, 

 without premising some general observations on the properties of light, 

 especially as regards its refraction, on which the phenomena of vision 

 are greatly dependent. 



1. LIGHT. 



The sun and the fixed stars are the great sources of light. It is 

 given off, also, from substances in a state of combustion, and from phos- 

 phorescent bodies ; and, by entering the eye directly, or after various 

 reflections or refractions, impinges on the optic nerve, and gives the 

 sensation of light. 



Two main opinions have been entertained regarding the nature of 

 light; the one, propounded by Newton that it consists of extremely 

 minute particles, emanating from luminous bodies ; the other that 

 of Des Cartes, Hook, Huygens, Euler, and others, that it is a sub- 

 tile, eminently elastic fluid an ether pervading all space ; the elastic 

 molecules of which, when put in motion by the oscillations of bodies, 

 impress the eye as sonorous vibrations affect the ear. It is not for 

 us to discuss this question of higher physics. We may merely re- 

 mark, that difficulties attend both hypotheses. According to that 

 of Des Cartes, it is not easy to explain, why an opaque body should 

 prevent the undulations from reaching the eye, or the change of 

 direction, which light experiences in passing from one medium into 

 another ; whilst, according to that of Newton, it is difficult to con- 

 ceive, how a luminous body, as the sun, can shed its immense tor- 

 rents of light incessantly, without undergoing rapid diminution ; and 

 how, with the extreme velocity of light, these particles should not be 

 possessed of sensible momentum ; for it has been found that a large 

 sunbeam, collected by a burning-glass, and thrown upon the scale of 

 a balance of extreme delicacy, is insufficient to disturb the equili- 

 brium. To the hypothesis of Newton it has been objected, that the 

 particles being reflected by thousands of bodies, and in innumer- 

 able directions, would necessarily jostle and interfere with each other. 

 This objection is not, however, as valid as it appears at first sight. It 

 will be seen hereafter, that the impression of a luminous object remains 

 upon the retina for the sixth part of a second. Admitting it, however, 

 to impress the eye for the 3 J^th part, three hundred particles, per 

 second, would be sufficient to excite a constant and uniform sensation 

 of the presence of light; and since, as we shall find, it traverses sixty- 

 seven thousand leagues in a second of time, if we divide this by three 

 hundred, we shall find a space of six hundred and seventy miles be- 

 tween each particle ; a distance equal to that in a straight line be- 

 tween New York and Savannah; and if we suppose six particles to be 

 sufficient per second, each will be separated from the other by a space 

 of thirty-three thousand five hundred miles ! 



Without deciding in favour of either of the great theories, that of 

 Newton admits of more easy application to our subject, and will, there- 

 fore, be employed in the various explanations that may be required. 



