264 THE POLARISCOPE IN MICROSCOPY [Ch. VIII 



position (solid nitrate of uranium and permanganate of potash in the 

 blu (MacMunn). The rule given by MacMunn is a good one: 



"The recognition of a body becomes more certain if its spectrum 

 consists of several absorption bands, but even the coincidence of these 

 bands with those of another body is not sufficient to enable us to 

 infer chemical identity, what enables us to do so with certainty is 

 the fact, that the two solutions give bands of equal intensities in the 

 same parts of the spectrum which undergo analogous changes on the 

 addition of the same reagent. It should be borne in mind that the 

 position of a band may be changed greatly through increased or 

 diminished dissociation, and that the absorption bands given by a 

 crystal may be quite different from those given by the same material 

 in solution and furthermore that the absorption spectra are usu- 

 ally different in different directions through the crystal" (Chamot, 

 p. 112). 



MlCRO-POLARISCOPE 



§ 421. The micro-polariscope, or polarizer, is a polariscope used 

 in connection with a microscope. 



The most common and typical form consists of two Nicol prisms, 

 that is, two somewhat elongated rhombs of Iceland spar cut diagonally 

 and cemented together with Canada balsam. These Nicol prisms 

 are then mounted in such a way that the light passes through them 

 lengthwise, and in passing is divided into two rays of plane polarized 

 light. The one of these rays obeying the ordinary law of refraction 

 is called the ordinary ray, the one departing from the law is called 

 the extraordinary rays. These two rays are polarized in planes at 

 right angles to each other. The Nicol prism totally reflects the ordi- 

 nary ray at the cemented surface as it meets that surface at an angle 

 greater than the critical angle, and only the less refracted, extraordi- 

 nary ray is transmitted. 



§ 422. Polarizer and analyzer. — The polarizer is the Nicol 

 prism placed beneath the object and by means of it the object is 

 illuminated with polarized light. The analyzer is the Nicol placed 

 at some level above the object, very conveniently above the ocular. 



When the corresponding faces of the polarizer and analyzer are 



