120 



APPENDAGES TO THE MICROSCOPE 



collar, which fits into the upper end of the tube (Fig. 66, b) 

 that receives the Polarizing 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 accom- 

 plished 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, 

 so that they may be used singly or in double or triple combinations ; 

 as many as thirteen different tints may thus be obtained. — When 

 the construction of the Microscope does not readily admit of the 

 connexion of the Selenite plate with the Polarizing prism, it is con- 

 venient to make use of a plate of brass (Fig. 68) somewhat larger 

 than the glass slides in which objects are ordinarily mounted, with 



Fig. 68. 



Selenite Object-carrier. 



a ledge near one edge for the slide to rest against, and a large circular 

 aperture into which a glass is fitted, having a film of selenite 

 cemented to it ; this ' Selenite Stage,' or object-carrier, being laid 

 upon the Stage of the microscope, the slide containing the object is 

 placed upon it ; and, by an ingenious modification contrived by Dr. 

 Leeson, the ring into which the Selenite plate is fitted being made 

 movable, one plate may be substituted for another, whilst rotation 

 may be given to the ring by means of a tangent-screw fitted into 

 the brass-plate.* 



90. Illuminators for Opaque Objects. — All objects through which 

 sufficient light cannot be transmitted to enable them to be viewed 

 in the modes already described, require to be illuminated by rays, 

 which, being thrown upon the surface under examination, shall be 



* An improvement on the ordinary Selenite Object-carrier, enabling 

 the Selenite plates to be changed without disturbing the object, has 

 been described by Mr. James Smith in "Quart. Journ. of Microsc. 

 Science," Vol. viii. (1860), p. 203 ; and he has more recently added a very 

 simple arrangement, by which rotation may be given to the object, 

 whilst the polarizing prism and selenite remain stationary (See "Transact, 

 of Microsc. Soc." Ser. 2, Vol. xiv. p. 101). — For an account of the nature 

 and properties of Polarized Light, which would be out of place in the 

 present treatise, see the chapters on that subject in Mr. Brooke's 

 "Manual of Natural Philosophy." 



