464 HISTORY OF OPTICS. 



which are polarized in opposite (that is, perpendi- 

 cular,) planes do not interfere at all 18 . Taking these 

 principles into the account, Fresnel explained very 

 completely, by means of the interference of undula- 

 tions, all the circumstances of colours produced by 

 crystallized plates ; showing the necessity of the 

 polarization in the first instance ; the dipolarizing 

 effect of the crystal ; and the office of the analyzing 

 plate, by which certain portions of each of the two 

 rays in the crystal are made to interfere and pro- 

 duce colour. This he did, as he says 19 , without 

 being aware, till Arago told him, that Young had, 

 to some extent, anticipated him. 



When we look at the history of the emission- 

 theory of light, we see exactly what we may con- 

 sider as the natural course of things in the career 

 of a false theory. Such a theory may, to a certain 

 extent, explain the phenomena which it was at first 

 contrived to meet; but every new class of facts 

 requires a new supposition, an addition to the 

 machinery ; and as observation goes on, these inco- 

 herent appendages accumulate, till they overwhelm 

 and upset the original frame-work. Such was the 

 history of the hypothesis of solid epicycles ; such 

 has been the history of the hypothesis of the 

 material emission of light. In its simple form, it 

 explained reflection and refraction ; but the colours 

 of thin plates added to it the hypothesis of fits of 

 easy transmission and reflection ; the phenomena of 

 18 Ann. Chim. torn. x. ' 9 Ib. torn. xvii. p. 402. 



