374 LECTURE XL. 



tion, ibid, xviii. Kelland on the Transmission of Light in crystallized Media, Camb. 

 Tr. vi. 323. On Reflection, Ed. Tr. xiv. 393 ; xv. 37, 511. On the Aggregate 

 Effect of Interference, Camb. Tr. vii. Ed. Tr. xv. 315. Green on Reflection, 

 &c. Camb. Tr. vii. 



LECTURE XL. 



<i 



ON THE HISTORY OF OPTICS. 



THE science of optics is not one of those which had been cultivated with 

 the greatest diligence and success by the philosophers of antiquity ; almost 

 every refinement relating to it has originated in the course of about two 

 centuries ; and some of its greatest improvements have been made within 

 these fifty years. The reflection of the rays of light is indeed an occur-, 

 rence too frequent and too obvious to have escaped the notice even of the 

 earliest observers : a river or a fountain was the first mirror ; its effect was 

 easily imitated by speculums of metal : and as soon as any philosophical 

 attention was paid to the phenomenon, it was easy to collect the equality 

 of the angles of incidence and reflection ; but although it was well known 

 that an oar, partially immersed in water, no longer appeared straight, it 

 was long before any attempts were made to ascertain the relation between 

 the angles of incidence and refraction. The Greeks were, however, 

 acquainted with the properties of the burning glass, which was sold as a 

 curiosity in the toy shops ; for it is well known, that one of the per- 

 sonages introduced by Aristophanes,* proposes to destroy the papers 

 [writing in wax] of his adversary by the assistance of this instrument. 

 The magnifying powers of lenses were, however, but little understood, 

 although it is scarcely credible that they could have escaped the notice of 

 a person in possession of a burning glass ; it appears from Seneca that the 

 Romans at least were informed of the effects of spherical refracting sub- 

 stances, and it is not improbable that some use was occasionally made of 

 them in the arts. 



Empedocles is perhaps the first person on record that wrote systemati- 

 cally on light. He maintained that it consisted of particles projected from 

 luminous bodies, and that vision was performed both by the effect of these 

 particles on the eye, and by means of a visual influence emitted by the eye 

 itself. Both of these doctrines were combated by Aristotle,t who thought 

 it absurd to suppose that a visual influence should be emitted by the eye, 

 and that it should not enable us to see in the dark ; and who considered it 

 as more probable that light consisted in an impulse, propagated through a 

 continuous medium, than in an emanation of distinct particles. Light, he 



* Nubes. 



t -De Sensu, ii. &c. But compare Meteor, i. 6 ; iii. 4, 5 ; and see authorities in 

 Kelland 's Lectures, p. 6. 



