438 A new Theory of Vision. 



light consists in undulations of an ethereal medium*. It may 

 be said, perhaps, that Huygens made use of these terms in con- 

 descension to the want of capacity in ordinary readers, who 

 would not have understood him had he explained himself in any 

 other way: but a philosophy which can be comi)rehended only 

 by a few, is rather metaphysical than solid; it can answer no ge- 

 neral purpose of usefidness, and, in all probability, is not founded 

 upon any solid principles. 



By supposing light to consist of separate particles, it is easy 

 to conceive how they may cross each other, as they are conti- 

 nually doing ; but there is no analogy in nature to support the 

 opinion that undulations of a fluid can cross each other without 

 one or the other, or both, being thrown into confusion. If we 

 throw two stones into w-ater at a little distance from each other, 

 two circles of undulations are produced, proceeding from two 

 centres; but the moment they meet tliey are broken and thrown 

 into confusion, and the undulations only proceed where there is 

 no interruption. 



If it is difficult to conceive how it is. possible for undulations 

 to cross each other without being broken, it is also equally 

 difficult to conceive how these undulations can pass through a 

 medium without being separated into particles, and then the hy- 

 pothesis is destroyed : a stone thrown into water easily passes 

 through, because the particles of water give way to the greater 

 force of gravity by which it is impelled; and a particle of light 

 raay do the same: but it is impossible for a fluid to pass through 

 another, \vithout one or the other being broken and dispersed; 

 and the water evidently is not, or we should see it in commotion. 

 If we hold a tumbler upside dou'n and immerse it in water, we see 

 tJiat the air still remains at the bottom of it, and cannot escape 

 through the water: And is it not, then, too much to suppose that 

 any other fluid can pass through, without either being thrown 

 into confusion or separated into particles ? 



I can readily conceive that particles of light and colour, en- 

 tering the eye in different directions, may impinge on the retina 

 distinct figures and colours, and that the mind may distinguish 

 these figures and colours by the sense of seeing, as it is commonly 

 understood, and not, as the philosophers will have it, by vibrations 

 on the nerve: but I can imagine nothing but confusion, when I 

 am told that light is nothing more than undulations of an ethereal 



* See the Eflinbur<;h Philosop'.ical Journal. In another part of it we 

 vead, " The phscnomena of inflection arc considered bj' M. Fresnel to be 

 inexplicable on the Newtonian theor}^ of the emission of luminous parti- 

 cles ; while almost all of them may be directly deduced by the Huygenian 

 theory of undulations.' I am fully persuaded myself, and I think 1 shall be 

 able to prove it, that inflection is a property that does not belong to light. 



medium j 



