EPOCH OF YOUXG AXD FRESXEL. 107 



polarized in opposite (that is, perpendicular,) planes do not interfere 

 at all. 19 Taking these principles into the account, Fresnel explained 

 very completely, by means of the interference of undulations, all the 

 circumstances of colors produced by crystallized plates ; showing the 

 necessity of the polarization in the first instance ; the dipohtriziit;-/ 

 effect of the crystal ; and the office of the analysing plaie, by which 

 certain portions of each of the two rays in the crystal are made to 

 interfere and produce color. This he did, as he says, 30 without being 

 aware, till Arago told him, that Young had, to some extent, antici- 

 pated him. 



When we look at the history of the emission-theory of light, we 

 see exactly what we may consider 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 incoherent appen- 

 dages 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 colors of thin plates added to it the hypothesis of fits of easy 

 transmission and reflection ; the phenomena of diffraction further 

 invested the particles with complex hypothetical laws of attraction and 

 repulsion ; polarization gave them sides ; double refraction subjected 

 them to peculiar forces emanating from the axes of crystals ; finally, 

 dipolarization loaded them with the complex and unconnected con- 

 trivance of moveable polarization ; and even when all this had been 

 assumed, additional mechanism was wanting. There is here no unex- 

 pected success, no happy coincidence, no convergence of principles 

 from remote quarters ; the philosopher builds the machine, but its 

 parts do not fit ; they hold together only while he presses them : this 

 is not the character of truth. 



In the undulatory theory, on the other hand, all tends to unity and 

 simplicity. "We explain reflection and refraction by undulations ; 

 when we come to thin plates, the requisite "fits" are already involve I 

 in our fundamental hypothesis, for they are the length of an undula- 

 tion ; the phenomena of diffraction also require such intervals ; and 

 the intervals thus required agree exactly with the others in magnitude, 



19 Ann. Chim. torn. x. 20 Ib. torn. svii. p, 402. 



