and its Use in Experiments of Polarization. 319 



In practice, the vise of Fresnel's rhomb in this part of the 



apparatus would (on account of its length) be attended with some 



inconvenience. I have therefore preferred for this purpose a plate 



of mica, of such a thickness that the ray polarized in the plane. 



i 13 5 



of one of its principal sections is retarded either -, -, or 7 of 



4 4* 4 



a wave (according to the convenience of splitting) more than that 

 polarized in the plane of the other. The mica being attached to 

 the unsilvered glass so that its principal section makes an angle 

 of 45° with the plane of reflection, an analyzer is produced which 

 answers the same purposes, in general, as that described above. 

 In strictness its effects are not the same, as the order of the 

 colours is in some cases sensibly disturbed. 



Upon trying this, when the incident light is circularly polarized, 

 the general effects are precisely such as were anticipated. Iceland, 

 spar exhibits rings without a cross of any kind : nitre, arragonite, 

 &c, exhibit the lemniscates uninterrupted in their whole extent, 

 and without any trace of hyperbolic brushes. Unannealed glass 

 exhibits dark patches surrounded by colours of different orders, 

 without any continuous brush. In these and in other cases that 

 I have tried, no alteration is produced in the appearances by turn- 

 ing the crystal, except that the system of rings, &c. is equally 

 turned. 



Perhaps the method of analyzation which I have described 

 may, with the application of circularly-polarized light incident, 

 be advantageously used for examining the nature of irregularly 

 crystalline bodies. For instance: the appearances presented by 

 unannealed glass in the common apparatus are singularly com- 

 plicated: but in this they are comparatively simple. In this the 

 eye sees at one glance (by the order of colour) how much lli< 



