and its Use in Experiments of Polarization. 317 



analyzer can transmit to the eye, and therefore the pole is seen 

 with full brightness. If it is the contrary, the light incident on 

 the analyzer is exactly of that kind which is totally suppressed 

 at the analyzer, and therefore the pole is perfectly black. 



The third conclusion is that (supposing, to fix our ideas, that 

 the direction of the light incident on the crystal is the same as 

 that which the analyzer can transmit) the intensity of light de- 

 pends only on the gain or loss of the ordinary or the extraordi- 

 nary ray : being at its maximum when that gain or loss is a whole 

 multiple of X, and nothing when the gain or loss is an odd 



multiple of 5. For the first of these propositions it is only 



necessary to state that the crystalline plate, having resolved the 

 incident light into two waves consisting of vibrations of different 

 kinds, and having retarded one set more than the other by a whole 

 multiple of X, unites the two sets again in exactly the same con- 

 dition in which they were at the resolution, and therefore they 

 emerge forming a kind of light which is exactly similar to the 

 incident light, and which is on that account susceptible of perfect 

 transmission by the analyzer. This applies to rpiartz as well as 

 to other crystals. For the second proposition we have only to 

 remark that when circularly polarized light is incident on Iceland 

 spar, nitre, and similar uniaxal and biaxal crystals, whatever be 

 the position of the planes of polarization, it is resolved into two 

 sets of plane vibrations at right angles to each other, one of which 



is - behind the other: and that when the relative path of these 

 waves is altered by an odd multiple of-, that which preceded 

 by - now follows by - (neglecting multiples of X) : and the direction 

 of the circularly-polarized light is thus reversed: and the emergent 



