244 PRACTICAL MICROSCOPY. 



light on the various starches maybe seen in Fig. 145. The 

 palates of molluscs are displayed remarkably well under 

 the influence of polarised light, especially when mounted in 

 gum dammar or Canada balsam. 



A very common object, seen in almost all collections of 

 slides, is the section of rhinoceros horn. When examined 

 by ordinary light it appears to resemble a bundle of hairs ; 

 but when viewed with the polariscope, each cylindrical 

 aggregation is marked by a cross, bearing a striking resem- 

 blance to the starch granules illustrated by Fig. 213. 



The muscular system of many minute animals can only 

 be satisfactorily made out by means of polarised light, 

 which very strikingly differentiates the tissues ; while such 

 objects as silk, cotton, flax, and jute are often to be distin- 

 guished from other substances by its aid. 



But by far the most important application of the micro- 

 scope and polariscope is in relation to petrology, whereby 

 much regarding the nature of most minerals may be satis- 

 factorily made out. With this end in view several petro- 

 logical microscopes have been produced during the past 

 few years, notably from the houses of Mr. Watson and 

 Messrs. Swift and Son, the latter making the instrument 

 shown in Fig. 214. 



This instrument is specially designed for the examination 

 of rocks and mineral substances, being furnished with a 

 concentric and revolving glass stage, self-centering arrange- 

 ment, goniometer, and complete polariscope. The polarising 

 prism is also provided with a divided circle and spring catch 

 to indicate when the Nicols are crossed ; it is made to slide 

 up in its fitting close to the under side of the object, and 

 on it are fitted condensing lenses of extreme high angle for 

 showing the brushes in crystals. When not required, the 

 polariser can be moved aside, entirely clear of the stage of 

 the microscope. The analyser is made to slide in the 



