THE APPARATUS FOR ROTATING THE OBJECTS. 317 



physiological nature which we do not purpose discussing here j 1 we 

 may, however, state that such an illusion be it greater or less is 

 unavoidable, and the only point of importance to us is, how under 

 these conditions the true azimuth of a microscopic image may be 

 most certainly determined. 



In most cases we should be approximately correct if we regarded 

 the line of bisection of the above-mentioned angle of 6 to 10 as 

 the measure. With normal (non-squinting) eyes, which undergo 

 approximately the same amount of turning on looking in the 

 direction of the axis of the Microscope, the possible error is 

 extremely slight. Where great accuracy is desired, this process is 

 not sufficient, but should be verified by other means. The follow- 

 ing methods may then be employed : 



(1.) If the eye-piece is provided with an aperture for the insertion 

 of the micrometer, a small rule, a strip of glass, or other suitable 

 object is slid in this aperture so as to cover the margin of the 

 object under investigation. The projecting portion then yields 

 the azimuth required, and it is easy to place an indicator upon 

 the graduation parallel with it. 



(2.) The micrometer-division, or the margin of a cover-glass 

 placed upon the diaphragm, is adjusted in the field of view parallel 

 to the direction in question ; the object is then removed, and in its 

 place a sufficiently long glass plate is laid, the margin of which, 

 appearing in the field of view as a dark line, is brought into the 

 same position determined by the micrometer. The production of 

 this margin then gives the reading of the azimuth upon the 

 graduated scale. If the analyser is fixed upon the body-tube, 

 it should be applied in such a manner that its position remains 

 unaltered on turning the eye-piece. 



Rotation round a Horizontal axis requires a special contrivance 

 adapted to lie upon the stage or to be screwed upon it. The 

 apparatus represented in Fig. 176 may be recommended. 



To the brass plate A B, having a large central opening, is 

 attached vertically a graduated semicircle, in the centre of 

 curvature of which the rotating-pin with milled head D is applied. 

 This pin carries on the other side of the semicircle two sprung 



1 The questions opened up here have recently been explained in several 

 ways; cf., for instance, the papers on this subject by Helmholtz in the 

 "Archivfiir Ophthalmologie," 1863 and 1864, and the older treatises quoted 

 therein. 



