THE ANALYSER. 311 



at each moment dependent thereon. Whether the loss be great 

 or small, the polarised portion of the reflected light is, under 

 all circumstances, polarised in the plane of reflexion, and therefore 

 passes through the prism undiminished, or is wholly absorbed 

 by it, according as the plane of polarisation of the extraordinary 

 ray coincides with the plane of reflexion of the mirror or is 

 at right angles to it. The position of the prism consequently 

 determines the polarised portion of the incident light, although 

 half the non-polarised portion is evidently transmitted in any 

 position. 



Theoretically regarded, Mohl's rule is therefore based upon facts. 

 As far as regards its practical value, we should not feel inclined to 

 estimate it highly, since the differences of intensity in the light 

 which are observed on turning the polariser, are always so very 

 slight that they may practically be disregarded. The reflexion at 

 the end face is of greater importance, inasmuch as the light 

 incident from above and from the side is not completely cut off. 

 In order to avoid this, it is well to turn the oblique terminal face 

 away from the source of light. 



2. THE ANALYSER. 



Of the various modifications of this instrument which may be 

 suggested, we will first consider the different positions it may 

 occupy with regard to the refracting surfaces of the Microscope. 

 Theoretically it is, of course, quite immaterial at what point above 

 the object the analysis takes place, provided it extends to all the 

 rays that reach the eye. The analyser may therefore be applied 

 above the eye-piece or immediately above the objective, or at any 

 point in the body-tube, or between the object and the objective, 

 provided its angle of aperture is sufficiently large. The question 

 is only what advantages or disadvantages are combined with these 

 different positions. 



If the analysing Nicol is situated immediately above the objective, 

 as in the Microscopes of Oberhasuser, then, according to Brlicke, 1 

 it is only possible to produce a perfectly dark field of view, with 

 rectangular crossing of the Nicols, by means of strong double- 

 refraction. According to this statement we lose in intensity of 

 1 " Denkschriften d. Wiener Akad." xv. (1858), p. 69. 



