THE ACTION OF CYLINDRICAL OBJECTS. 



353 



Secondly, we take the case where the cylinder cannot be split 

 up on account of its minuteness, or from other causes, and that 

 hence we can only obtain longitudinal views of the whole cylinder. 

 It may be foreseen that such views, in most cases, replace the 

 diametral longitudinal sections ( B, Fig. 202), the effect of light 

 which the two marginal zones produce being essentially the same. 

 Their agreement will be the more complete, the more we diminish 

 the refraction of the light which takes place on entrance into the 

 substance of the cylinder. If we are 

 successful in completely eliminating this 

 refraction by proper selection of the sur- 

 rounding medium (glycerine, oil, &c.), the 

 rays incident from below act just as 

 in a very thin plate i.e., they attain the 

 same differences of path next to the 

 margin, as if the middle piece B B (Fig. 

 203) alone were active. Further inwards, 

 the gradually increasing effects of the 

 triangular pieces s and t are added ; 

 though it is clear that they in no case 

 preponderate in the peripheral layers 

 of the cylinder. The two marginal zones must therefore, 



FIG. 203. 



position of the elements of the cylinder, and then explains the appearance of 

 the dark cross in the section as the "three principal directions of the optic 

 axis : " the perpendicular, the tangential, and the radial. To the perpendicular 

 position of the optic axis the "cross of the first order" corresponds; to the 

 tangential position the " cross of the second order ; " and to the radial position 

 the " cross of the third order." And it is remarked that there are preparations 

 from the vegetable kingdom (albumen of Phytelephas] in which crosses of the 

 first, and others of the third order, appear side by side in one and the same 

 section. Against this statement we must first of all observe that the assump- 

 tions themselves, so far at least as vegetable preparations are concerned, are at 

 variance with reality, inasmuch as all the membranes of cells that are 

 accurately known (it is true there are but few) have been found to be optically 

 biaxial. But even disregarding this, the "cross of the first order" is a 

 physical impossibility under the polarising Microscope (especially with amplifi- 

 cations such as are necessary with most of the objects ofithis class), for with a 

 perpendicular axis the whole section appears dark. "We must emphatically 

 remark that by a polarising Microscope we understand an ordinary Microscope 

 fitted with polarising prisms the sole instrument, moreover, by which the 

 microscopically small objects (such as bast-cells, vegetable albumen, &c.), 

 mentioned by Yalentin himself, can be observed. The image which an instru- 



A A 



