174 * DISSERTATION SECOND. [part i. 



transparent. The reason for calling light an act of a 

 transparent body is, that, though a body may be transpa- 

 rent in power or capacity, it does not become actually 

 transparent but by means of light. Light brings the trans- 

 parency into action ; it is, therefore, the act of a transpa- 

 rent body. In such miserable puerilities did the genius of 

 this great man exhaust itself, owing to the unfortunate di- 

 rection in which his researches were carried on. 



In his farther speculations concerning light, he denied 

 it to be a substance; and his argument contains a singular 

 mixture of the ingenious and the absurd. The time, he 

 says, in which light spreads from one place to another is 

 infinitely small, so that light has a velocity which is infinite- 

 ly great. Now, bodies move with a velocity inversely as 

 the quantities of matter which they contain ; light, there- 

 fore, cannot contain any matter, that is, it cannot be ma- 

 terial. ' That the velocity of light was infinitely great, 

 seemed to him to follow from this, that its progress, esti- 

 mated either in the direction of north and south, or of east 

 and west, appeared to be instantaneous. In the opinion 

 of the Platonists, and of the greater part of the ancients, 

 vision was performed by means of certain rays which pro- 

 ceeded from the eye to the object, though they did not 

 become the instruments of conveying sensations to the 

 mind, but in consequence of the presence of light. In this 

 theory, we can now see nothing but a rude and hasty at- 

 tempt to assimilate the sense of sight to that of touch, 

 without inquiring sufficiently into the particular characters 

 of either. 



Epicurus, and the philosophers of his school, as we learn 

 from Lucretius, entertained more correct notions of vision, 



1 The truth of the mathematical proposition, that -^ = 0, 

 was perceived by Aristotle. A strong intellect is always visi- 

 ble in the midst of his greatest errours. 



