3l8 ACCESSOKY APPARATUS 



the rays before they pass through the object, and to apply to them, 

 in some part of their course between the object and the eye, an 

 analysing medium. These two requirements may be provided for 

 in different modes. The polariser may be either a bundle of plates 

 of thin glass, used in place of the mirror, and polarising the rays by 

 reflexion ; or it may be a ' single image ' or ' Nicol ' prism of Iceland 

 spar, which is so constructed as to transmit only one of the two 

 rays into which a beam of ordinary light is made to divaricate by 

 passing through this substance. Of these two methods the * Nicol ' 

 prism is the one generally preferred, the objection to the reflecting 

 polariser being that it cannot be made to rotate. This polarising 

 prism is usually fixed in a tube, and is shown in a simple form in 

 A, fig. 261 ; it is usually employed in a sub-stage which rotates by a 

 rack-and-pinion arrangement, so that rotation of the prism is easily 

 effected. For the analyser a second ' Nicol ' prism is usually em- 

 ployed ; and this, fixed in a short tube, may be fitted into a collar 

 interposed between the lower end of the body and the objective, 

 as is shown in B, fig. 261. The prism in this fitting can also 



be rotated by the fingers 

 grasping and giving circular 

 motion to the inner fitting of 

 B, arid it is always important 

 that the polarising prism 

 should be large, so as not to 

 act as a diaphragm to the con- 

 denser, thus cutting off the 

 light when it is used ; for the 

 polarising apparatus may be 

 FIG. 261. Polarising apparatus. worked in combination either 



with the achromatic con- 

 denser, by which means it may be employed with high-power 

 objectives, or as a ' dark-ground ' illuminator, which shows many 

 objects such as the horny polyparies of zoophytes gorgeously 

 projected in colours upon a dark field. 



For bringing out certain effects of colour by the use of polarised 

 light it is, as already stated, desirable to interpose a plate of selenite 

 between the polariser and the object ; and it is advantageous that 

 this should be made to revolve. A very convenient mode of effecting 

 this is to mount the selenite plate in a revolving collar, which fits 

 into the upper end of the tube that receives the polarising prism. 

 In order to obtain the greatest variety of coloration with different 

 objects, films of selenite of different thicknesses should be employed ; 

 and this may be accomplished by substituting one for another in the 

 revolving collar. A still greater variety may be obtained by mounting 

 three films, which separately give three different colours, in collars 

 revolving in a frame resembling that in which hand-magnifiers are 

 usually mounted, this frame being fitted into the sub-stage in such 

 'a manner that either a single selenite, or any combination of two 

 selenites, or all three together, may be brought into the optic axis 

 above the polarising prism (fig. 262). As many as thirteen different 

 tints may thus be obtained. When the construction of the micro- 



