FOK EXJUBITING THK I'OLAIUZATION Of LIGHT. 91 



between the condensing-lens and the minor, /and g, being placed for- 

 wards, there is seen a cooled cube upon tlie mirror darkened by the ana- 

 lysing prism, fig. 6. ; and consequently when the cube is turned 45°, 

 fig. 7, the same phsenomena are observed as if both the mica plates 

 had been removed. Between the two mica-plates, whose axes cross 

 each other at right angles, appears fig. 8, and indeed unchanged when 

 the tube is turned in its ring. Fig. 9. is the complementary figure to it, 

 which is obtained by turning the analysing prisni to 90°, without 

 changing the position of the mica plates, lif is bent backwards, there 

 appears the modification of the linear figure, which produces circu- 

 larly polarized incident light linearly analysed. 



Of this as well as of that modification produced by circular analysis of 

 the linear light which follows when the cube is close to the condensing- 

 lens, it is easy to form an idea, by imagining the linear figure divided 

 into four equal quadrants by two perpendicular lines, and the even 

 quadrants removed from the central point about ^ interval, and the 

 odd ones approaching to within the same distance ; or vice versa, these 

 removed, whilst those are made to approach. To polarize lamp-light 

 by reflexion, the better way is to fix upon the condensing-lens (itself 

 capable of revolving,) a mirror inclined at the fixed angle of polarization. 

 If, by means of the polarizing prism, the instrument has before been 

 placed upon the lamp, after the prism is turned aside and the mirror is 

 fixed, that instrument, without changing its inclination, is turned round 

 its perpendicular stand, until the system of rings is seen anew in the Ice- 

 land spar within the ring. Instead of employing Nicol's prisms, the light 

 may be polarized by absorption in tourmaline plates, or by successive 

 refractions through a series of glass plates. These are screwed into the 

 stands in similar frames. 



In order to obtain the deviation of the plane of polarization by 

 refraction, the refracting bodies are introduced into the stand s^. The 

 deviation by reflexion may be conveniently observed by turning the 

 rod at an angle toward a definite point. As, however, this (experiment 

 is easily made in another manner, I thought it unnecessary to com- 

 plicate the apparatus for it. In the same manner the apparatus may be 

 changed into a polarizing microscope, with a still larger field of view, 

 by the addition of some lenses and stands. But as this will be desii^- 

 ble in very few experiments, besides that the construction of such an 

 apparatus by means of single rings fitting one another is easy, I omit- 

 ted them in this instrument. 



When a glass warming or cooling is to be examined in the polarized 

 light, the prismatic rod is so inserted into the frame h that one of its 

 faces which have hitherto formed the sides, is brought into a horizon- 

 tal position below. All the stands are then at the side of the horizon- 

 tal rod turned to 120", which presents no obstacle to the heating by 



