POLARIZATION OF LIGHT. [ C12 ] POLARIZATION OF LIGHT. 



form beautiful microscopic objects as 

 horse-hair, portions of feathers, sections of 

 quill, of hoof. horn, fish-scales (salmon), 

 &c. 



The influence of vegetable structures on 

 polarized light has been thoroughly investi- 

 gated by Mohl, whose interesting account 

 we are able to confirm, and a brief notice of 

 it is desirable here; but the observations 

 apply equally to the more feebly anisotro- 

 pous tissues of animals. As it is desirable 

 to obtain as much light as possible, a glass 

 prism is preferable to the ordinary mirror 

 for illumination ; Nicol's prisms are better 

 than tourmaline or Herapathite for the pola- 

 rizer and analyzer ; and the latter should 

 be as large as possible. Further, the light 

 emerging from the polarizer should, if 

 possible, be condensed by an achromatic of 

 large aperture ; or the condensation may be 

 effected by a hemispherical flint-glass lens, 

 5 lines in diameter, havdng its plane face 

 turned towards the object. The object- 

 glasses must be of large angular aperture ; 

 a power of 4-10" is sufficient for most 

 objects; but 1-4", and even 1-8" object- 

 glasses may be made to transmit sufficient 

 fight. It is requisite to provide plates of 

 the doubly-refracting substances mica and 

 gypsum, mounted so that they can be in- 

 serted between the polarizer and the con- 

 denser, and revolved horizontally while so 

 placed. Those of mica are used for de- 

 tecting weak degrees of doubly-refracting 

 power, being of such thickness as to give a 

 grey field with a white or black object 

 when the prisms cross. The thin laminae, 

 of which six may be provided, from the 

 thinnest possible up to 1-20'", should be 

 cemented with Canada balsam between 

 glass plates. For obtaining colours, plates 

 of gypsum, similarly mounted, are best. 

 Mohl prefers such as give a red field, and 

 provides plates of different thickness, giving 

 the reds of the different orders of Newton's 

 rings. 



It is easy to ascertain whether an organic 

 body shows positive or negative colours, 

 by comparing its colour, when seen with a 

 plate of gypsum in a certain definite posi- 

 tion, with' the colour given under the same 

 circumstances by a strip of glass brought 

 into a state of tension by slight bending, or 

 with the colours of a suddenly-cooled glo- 

 bule of glass. In this way the author de- 

 termined that the fibres of a spiral vessel 

 displayed negative colours, and the laminae 

 of a starch-corpuscle positive colours, and 



then applied these organic structures, by 

 comparison, for ascertaining the properties 

 of other objects. The objects to be exa- 

 mined should be mounted in a liquid or 

 other substance rendering them as trans- 

 parent as possible, such as glycerine, Canada 

 balsam, or an essential oil. 



When ordinary globular or cylindrical 

 cellular tissues are viewed by cross sections, 

 their substance is seen to be doubly refrac- 

 tive ; for when the prisms cross, the circular 

 sections of the cell-walls appear like rings 

 of bright light on a black ground, but with 

 the ring divided into four quadrants by dark 

 stripes, as if a black cross lay over it; when 

 the prisms are placed parallel, the parts of 

 the section previously bright appear dark, 

 and vice versa, on a bright field. If a sec- 

 tion of polyhedral cellular tissue is viewed 

 in the same way, the appearances are 

 somewhat different, since the cut edges 

 are here straight lines, variously inclined 

 towards the prisms ; those which are per- 

 pendicular to the prisms are invisible, 

 while those standing obliquely are bright 

 in their whole length. In general, cell- 

 membrane acts more powerfully on the 

 light the denser its substance, and soft col- 

 lenchymatous tissues are far less powerfully 

 doubly-refractive than wood-cells. When 

 the cells have the walls much thickened, it 

 is common for the primary cell-membrane 

 to be much more powerfully refractive than 

 the secondary layers. The' influence of cel- 

 lulose membranes upon polarized light is 

 not much affected by bleaching them with 

 nitric acid and chlorate of potash (Schultze's 

 reagent). It has been supposed that the 

 remarkable effect produced by the epidermis 

 of Equisetum hyemak is attributable to the 

 silex present; but Mohl finds the action 

 greatly weakened by destroying the organic 

 matter by a red heat. But this heating 

 does not remove the power there, nor in 

 the Diatomaceae, of which Mohl confirms 

 Bailey's statement, in contradiction to Eh- 

 renberg, that various species of Navicula, 

 Synedra, Pletirosigma, and Melosira are de- 

 cidedly doubly refractive. 



Very remarkable phenomena are pro- 

 duced when the polarized light is made to 

 pass through plates of mica or selenite. In 

 the first place, thin plates of mica often 

 allow of the discovery of a doubly-refracting 

 power too feeble to be detected by the 

 prisms alone the degree of illumination of 

 the object being slightly different from that 

 of the field on which it is viewed. But the 



