1 8 HOW TO WORK 



had regarded one set of characters only. And by employing 

 polarised light peculiarities in the structure of an object may be 

 discovered which cannot be perceived when it is examined by 

 ordinary light. 



I must, therefore, draw the student's attention to the three 

 following methods of directing the light upon objects to be submitted 

 to microscopical examination. 



1. Reflected light. In the examination of an object by reflected 

 light, ordinary diffused daylight may be allowed to fall upon it, or 

 light may be received upon a metallic reflector, or be refracted 

 through a prism placed at the proper angle and thus made to impinge 

 upon the surface of the object. The intensity of the illumination 

 may be increased by employing a concave mirror or a bull's-eye 

 condenser, 27. 



By this mode of examination we gain information concerning the 

 peculiarities of the surface only, as in looking at objects under 

 ordinary circumstances. The surface of a perfectly transparent 

 object may be examined by reflected light, 30, 32. 



2. Transmitted light passes through the object which is examined, 

 which must, therefore, be transparent or capable of being rendered 

 so by some special method of preparation, 140. 



In this way any peculiarities of internal structure are discerned. 

 Transmitted light may be made to pass from the source of illumina- 

 tion direct through the object, or the rays of light may first be 

 received by a mirror or prism and then transmitted in a straight or 

 oblique direction through the preparation. 



3. Polarised Light. The light is polarised by being made to 

 traverse certain crystalline substances which possess the polarising 

 property before it is transmitted through the object. A crystal of 

 that form of carbonate of lime, known as Iceland or rhomboidal spar, 

 tourmaline, or iodo-quinine, is the most convenient for this purpose. 

 The first is generally used under the name of Nicol's prism which is 

 made by dividing a crystal of Iceland spar obliquely, and then care- 

 fully cementing the two portions together with Canada balsam. In 

 this way one of the two images produced by this double refracting 

 crystal is refracted out of the field of vision while the polarising 

 property is not in any way affected. Dr. Herepath has sent me two 

 beautiful crystals of the iodo-quinine or herapathite which he dis- 

 covered some years ago. The crystals are mounted between two 

 pieces of thin glass and work very satisfactorily. One of the 

 crystals above referred to is fitted beneath the stage of the micro- 

 scope. This is called the polariser. Another termed the analyser 

 is inserted in the tube of the microscope or is placed above the eye- 



