3i8 ACCESSORY APPARATUS 



the rays In-fore they pass through the object, and to apply to them, 

 in soiiii- part of ilit-ir course between the object and the eye, an 

 ,i<iliiiii<i iiiediuui. These two requirements may lie provided for 

 in different modes. The polarise/- 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 lie a single image ' or ' Nicol ' prism of Iceland 

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

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

 passing through this sub>tance. 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. tig. 261 ; it i> 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 (ntnlt/xt'r a second ' Nicol ' prism is usually em- 

 ploved : and this, tixed in a short tube, may lie 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, and 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 mav be 

 Fie.. :><ii. Polarising apparatus. worked ill 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 selenitr 

 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. 

 I n order to obtain the greatest variet\ of coloration with different 

 objects, films of >elenite of different thicknesses should be employed ; 

 and this may In- accomplished by substituting one for another iii the 

 re\ol\ in- collar. A stillgreater variety may I ..- obtained by mounting 

 bhree films, which separate!} give three different colours, in collars 

 revolving in a frame resembling that in which hand magnifiers are 

 usualK mounted, (hi- frame hein- fitted into the sub-stage in such 

 a manner thai either a single .selenite. or any combination of two 

 or all three together, may IK- brought into the optic axis 

 be polarising prism (fig. 262). As many as t hi rteeii different 

 thus be obtained. When the construction of the micro- 



