DETERMINATION OF THE AXES OF ELASTICITY. 333 



ellipsoid. The differences of direction which occur, therefore, relate 

 only to their position within one plane i.e., to the angles which the 

 axes form with the longitudinal and latitudinal directions of the 

 cell. Accordingly, in sections cut perpendicularly to the surface of 

 the membrane, one axis of the effective ellipse of elasticity lies in 

 the direction of the layers, the other is at right angles to it, and is 

 at the same time an axis of the ellipsoid. This obtains likewise 

 with the membranes of cellular tissues, so far as we know them ; 

 here, however, observation is rendered more difficult by the circum- 

 stance that the two lamellae, of which the dividing walls consist, 

 really represent two objects, whose axes of elasticity are possibly 

 very differently situated. 



The second point that comes into consideration in the deter- 

 mination of the ellipsoid of elasticity, viz., the relative magnitude 

 of the axes, assumes, under certain circumstances, a more exact 

 acquaintance with the phenomena of polarisation. It is of pri- 

 mary importance that the observer should be familiar with the 

 scales of the interference colours, at least up to the third or 

 fourth order, which appear on gradual increase of the difference 

 of path between the ordinary and the extraordinary rays, and 

 that he should be quite clear about the factors which produce 

 the variation of the tints. We must refer those who are not 

 conversant with the subject to the text-books of Physics, since a 

 detailed exposition of these phenomena here would lead us too far 

 from our subject. We consider it, how- 

 ever, within our province to present the 

 more important points in their mutual 

 connection, and to elucidate their relations 

 to the ellipsoid of elasticity. 



If A B (Fig. 190) is a parallelepiped of 

 glass compressed in the directions of the 

 arrows for example, one end of an object- 

 slide then the position of its effective 

 ellipse of elasticity is represented in the 

 figure. An object-slide of this sort ap- FlG - 19 - 



pears, on rectangular crossing of the 



Nicols, most brightly illuminated when it forms an angle of 

 45 with the planes of polarisation, and is consequently situated 

 in a diagonal position. It then produces in white light an 

 interference colour, which, with given thickness, increases in pro- 



