FOR EXHIBITING THE POLARIZATION OF LIGHT. 91 
between the condensing-lens and the mirror, f and g, being placed for- 
wards, there is seen a cooled cube upon the mirror darkened by the ana- 
lysing prism, fig. 6.; and consequently when the cube is turned 45°, 
fig. ’7, the same. phenomena 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 prism to 90°, without 
changing the position of the mica plates. If f 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 versd, 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 aseries 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 convehiently 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 sonie lensés and stands. But as this will be desira- 
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 thé frame / 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 
