ILLUMINATION IN CONNEXION WITH POLARIZATION. 173 



motion, and eacli one of these again reproduces all the others, so 

 that a light-ray is not an isolated force, but is one of an association 

 of forces, of which illumination is only one of the effects ; but of 

 this group, the first five out of the six are effective solely by virtue 

 of polar action, heat and light, in becoming polar ; electricity, 

 magnetism, and chemical action, in losing polarity and becoming 

 passive. It is thus the polarity induced by the impact of the ray 

 which excites, or confers upon the reflected or the refracted portion of 

 the ray, a condition enabling it to convey the impression of the object 

 to the eye, and the desideratum is to restrict the effect as much as 

 possible to this one action. 



" If light reflected from the surface of almost any except metallic 

 bodies, be examined with a plate of Tourmaline, it will, in general, 

 be found to show traces of polarization. . . Every ray of ordinary 

 light incident at any angle upon a transparent plate, is partly reflected 

 and partly refracted : the reflected ray is partially polarized, and so 

 also is the refracted ray. . . . Eeflection is generally, perhaps al- 

 ways, accompanied by refraction." There is, however, one particular 

 angle at which polarization is at a maximum. This angle is not the 

 same for all substances, as it varies according to their refractive 

 power ; hence, for water it is 52° 45'; glass, 51° 35'; amber, 56° 

 35'; Iceland spar, 58° 23' ; and for the diamond, 68° 02'. 



When light is incident at the polarising angle, the reflected and 

 the refracted rays make together a right angle, while the tangent of 

 the incident ray equals the refractive index of the substance em- 

 ployed, so that this tangent subtracted from 90° gives its angle of 

 polarization, and it has been assumed that the angle for amber, or 

 about fifty-six degrees, will be about the angle for the majority of 

 organic bodies viewed with the microscope. 



In the illumination of an opaque object by means of a side lens, 

 or a metallic reflector, a very satisfactory light is generally ob- 

 tained almost as a matter of course ; this, however, is to be ac- 

 counted for by an approximation to the polarizing angle happening 

 to present itself as the most convenient position available. But should 

 the Lieberkuhn be substituted for the side illumination, the very 

 reverse of this becomes a natural consequence. The mass of direct 

 rays which are thrown down, drowning the object with vertical light 

 that obliterates all detail, is an abuse, and not its legitimate use, and 

 has done much to prevent the instrument from being appreciated 

 as it deserves. If it be arranged, as shown in Fig. 3, so as 



