314 Professor Airy on a new Analyzer, 



same as before it entered. The use of the analyzing plate is to 

 resolve the emergent light according to some general law into 

 two parts, of which one is totally suppressed and the other (at 

 least a definite part of it) is wholly reflected. to the eye. As the 

 crystalline plate has at incidence resolved the incident light into 

 two parts which at emergence it has united with different degrees 

 of retardation according to the direction in which the light was 

 incident, the nature of the emergent light is different according to 

 the direction in which it was incident, or (which is the same) 

 according to the direction in which it emerges: and when resolved 

 according to the general law above mentioned, the proportion 

 of the reflected part varies according to that direction : and hence 

 the various intensities of the light in different parts of the image 

 seen with the ordinary apparatus. And this may be considered 

 as the general explanation of the use of an analyzer, including in 

 this term the common analyzing plate as well as such combinations 

 as will be hereafter described. 



By the common analyzing plate (an unsilvered glass reflector 

 at the polarizing angle, or a plate of tourmaline, or a doubly re- 

 fracting prism considered with reference to one pencil only) the 

 emergent light is resolved (according to Fresnel's theory) into two 

 sets of vibrations, one parallel to and the other perpendicular to 

 the plane of polarization of the analyzing plate; the former of 

 these is wholly suppressed, and the latter is wholly transmitted to 

 the eye. And this is the oidy kind of analyzation, so far as 

 I know, that experimenters or theorists have ever yet considered. 

 But there is not the least need for confining ourselves to this 

 method of analyzation. There are other methods, quite as simple 

 in theory, and nearly as easy in practice, which effect a resolution 

 of a very different kind. As the first among these I may mention 



