118 APPENDAGES TO THE MICROSCOPE. 



devised. It is open, however, to certain objections, which become 

 apparent when very high powers are used and difficult objects are 

 under examination ; and to obtain the most perfect white-cloud 

 illumination possible, is the object of an apparatus devised by Mr. 

 Gillett. This consists of a small camphine lamp, placed nearly in 

 the focus of a Parabolic Speculum, which reflects the rays either at 

 once upon a disk of roughened Enamel, or upon a second (hyperbolic) 

 Speculum which reflects them upon such a disk. A very pure and 

 concentrated light is thus obtained ; and as the forms of the 

 incident pencils are broken up by the roughened surface, that 

 surface takes the place of a lamp as the source from which the 

 rays primarily issue. The advantage of this illumination is 

 specially felt in the examination of objects of the most difficult 

 class under the highest powers. 



88. Polarizing Apparatus. — In order to examine transparent 

 objects by Polarized Light, it is necessary to employ some means of 

 polarizing 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 analyzing medium. These two requirements may be 

 provided for in different modes. The Polarizer may be either a 

 bundle of plates of thin glass, used in place of the mirror, and 

 polarizing 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 ; or 

 it may be a plate of Tourmaline, or one of the artificial tourmalines 

 composed of the disulphate of iodine and cpainine, now known by 

 the designation of ' Herapathite ' after the name of their inventor. 

 Of these methods, the ' Nicol' prism is the one generally pre f erred, 

 the objection to the reflecting polarizer being that it cannot be 

 made to rotate ; the Tourmaline is undesirable, on account of the 



Fig. CG. 

 A B 





Fitting of Polarizing Prism in Smith and Beck's Microscope. 



colour which it imparts when sufficiently thick to produce an 

 effective polarization ; whilst the crystals of Herapathite are 



