MICROSCOPES. 209 



In such cases the use of polarised light without an analyser 

 may be very useful. You. use it simply as light, and rotate 

 the prism so as to get the plane of polarisation in different 

 directions. The more common plan is to use it with an 

 analyser placed generally over the eye-piece. By certain 

 adjustments, when no object is on the stage, the whole field 

 of the microscope is dark; but if you place under the 

 microscope objects which have more or less powerful double 

 refraction, they depolarise the light and appear either white 

 or coloured, dr invisible, according to the angle at which 

 they are placed in relation to the plane of polarisation. In 

 order to study these effects, either the object or the plane 

 of polarisation must be rotated. Time will not permit of 

 entering further into it, but I may just draw your attention 

 to one or two illustrations. Some objects may show little 

 or no structure when examined with ordinary light, but by 

 using polarised light various dark markings and colours may 

 be seen, which vary as you rotate the plane of polarisation. 

 By careful induction you may form a very accurate opinion 

 as to the kind of structure before you. Thus, if the object 

 give a well-defined black cross rotating with the plane of 

 polarisation, it indicates that small crystals radiate uniformly 

 from a single centre ; whereas, if there be an irregular varying 

 distribution of dark patches shading off into bright or coloured 

 portions, all changing gradually as the plane of polarisation is 

 rotated, there must be an irregular grouping of imperfectly 

 radiating crystals. On the other hand, if on rotating the 

 plane of polarisation the object becomes uniformly dark and 

 bright, you know it is a portion of one simple crystal. A 

 great many other most interesting facts may be made out in 

 that manner by using polarised light. Such general con- 

 clusions are more simple and obvious in the case of mineral 

 structures, but are by no means confined to them. Much 

 may thus be learned respecting the arrangement of the 

 mineral matter in the various kinds of calcareous shells and 

 other organisms, and I cannot but think that much remains 

 to be learned even in the case of more purely organic 

 structures. 



When we use very high powers 1,000 linear, or upwards 

 and employ object-glass of very short focal length, we must 

 contend with another class of difficulties quite distinct from 

 any I have hitherto mentioned. In using very high powers 



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