472 



LECTURE XL 



ON THE HISTORY OF OPTICS. 



A HE 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 occurrence 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 ea5ily 

 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, partial- 

 ly 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 personages, introduced by Aristophanes, 

 proposes to destroy the papers of his adversary by the assistance of this in- 

 strument. 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 

 substances, 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 systematically 

 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 



