NORRENBERG'S "POLARISING MICROSCOPE.' 



373 



ance with this we get with selenite plate red I., for example, 

 yellow above and below, blue to the right and left, and, if we 

 invert the minute discs, blue above and 

 below, and yellow to the right and left. 

 With horizontal position of the axis we 

 do not observe any double-refraction. 



From this action it follows, in the 

 first place, that all crystal needles are 

 situated exactly as if we had led the same 

 needle round in a circle in a plane at 

 right angles to the axis ; and further, that 

 no axis of the ellipsoid of elasticity coin- FlG 



cides with the radius. Various possible 

 cases are, of course, imaginable as to form and position of the 

 ellipsoid of elasticity. 



YIL 



ON THE EMPLOYMENT OF NOEEENBEEG'S 

 POLAEISING MICEOSCOPE." 



WE devote a few remarks to Norrenberg's "Polarising Micro- 

 scope," the optical arrangement of which we must here assume 

 to be essentially known. Valentin says of this instrument that 

 in many cases it helps us in researches in polarised light 

 where the ordinary Microscope would no longer be sufficient. 

 We cannot agree with this opinion. Our conviction is that even 

 the best constructed polarising apparatus is of no exceptional 

 importance for microscopic purposes. Such apparatus is, 

 according to its construction, only applicable in cases where 

 the object to be investigated acts as a plate of crystal, and 

 where it, moreover, fills the whole field of view. The powerful 

 lenses with which the Norrenberg instrument is provided certainly 

 make it possible to reduce the field of view to *1 mm. ; but the 

 condition always remains that only those rays reach the eye 

 which have undergone the double-refracting influence of the 

 object. The lens serving as eye-piece is indeed adjusted to a 

 plane which coincides with the posterior focal plane of the 



