THE ACTION OF CYLINDRICAL OBJECTS. 359 



selenite plates, neutral longitudinal stripes at once appear, which 

 form the transition between the opposite colours of the marginal 

 and surface views, and which therefore 

 appear without selenite as dark lines 

 (Fig. 206). These lines represent 

 the places where the resulting ellipse 

 of elasticity of the surface view 

 (which, of course, gradually passes 

 over on both sides into the one of 

 the opposite margin) has attained a 

 circular form that is, where the 

 superposed ellipses of the half of the 

 cylinder, turned towards and from, j, 20g 



intersect at right angles. A rectan- 

 gular intersection, however, presupposes, on account of symmetrical 

 position, that the two ellipses are inclined less than 45 to the 

 axis of the cylinder. 



If the question were considered from a purely geometrical point 

 of view, we might suppose that, from the known distance of the 

 neutral lines from the margin, we could determine the unknown 

 angle e, which the superposed ellipses form in the median zone ; 

 for it is obvious that the distance from the middle line, at which 

 the above-mentioned inclination of 45 is reached, is the greater 

 the more the inclination to the middle line deviates from it. But 

 since each layer in the cylinder has its own neutral line, which 

 appears laterally displaced with regard to the adjoining layers, and 

 more or less illuminated by their interference colour, the combined 

 effect does not appear sufficiently decided to be of practical value 

 in the way we have indicated, on account of the minuteness of 

 the objects. The same is also true of the determination of the 

 optic axes, where = 0. It will suffice then to call attention in 

 general to the rather complicated relations which in this case 

 arise. 



The neutral longitudinal stripes appear most perfectly in 

 thick- walled, hollow, cylindrical cells, which refract the light 

 strongly ; yet with increased thickness also they are tolerably 

 distinctly visible. We have observed this especially in vegetable 

 hairs (e.g., of Stocky s), but here and there the tissue-cells also 

 furnish good examples. 



A second case, which to a certain extent forms the counterpart 



