INTRODUCTORY CHAPTER. 9 



ed to the object-glass, and from it the rays are reflected downwards and brought to 

 a focus upon the object itself. 



THE CAMERA LUCIDA. This instrument is fitted to the eye-piece of the micro- 

 scope, and enables the observer to sketch upon a paper placed upon the table the 

 magnified image of an object seen in the microscope. In Nachet's camera the in- 

 strument consists of a triangular glass prism, having its three faces and angles equal. 

 Fig. 4 shows this instrument mounted in a cap which fits the top of the eye- 

 piece. 



To the accessary apparatus already mentioned may be added the animalcule 

 cage, the machine for cutting circles of the glass, dissecting-scissors, and knives ; 

 glass- tubes, needles, the metallic stage, spirit-lamp, and hand forceps; the compres- 

 sor and the machine for cutting sections of wood, and some others. 



POLARIZED LIGHT. 



Without entering into the subject of polarized light, it is here sufficient to 

 say, that when certain objects are viewed under the microscope by polarized light 

 instead of common light, they glow with the most splendid and gorgeous colors. 

 In order that the light employed may be polarized a special apparatus is employed, 

 which can be readily attached to the microscope. This apparatus consists of a 

 Nichol's prism, placed below the stage of the microscope, and called a polarizer ; 

 and an analyacr, which is usually also a Nichol's prism set in abrass tube, and insert- 

 ed in the body of the microscope directly behind the object-glass. 



MOUNTING OF OBJECTS. 



As the microscopist not merely desires to prepare objects for present exami- 

 nation, but also to preserve them for future inspection, various modes have been 

 adapted to effect this end, according to the different nature of the objects to be pre- 

 served. These modes are included under the general term of " mounting, 11 Trans- 

 parent objects are mounted upon slips of clear glass, 3 inches by 1 inch, or 3 inches 

 by 1-| inch. There are three modes of mounting transparent objects, viz., the 

 dry way; in some preserving fluid ; and in Canada balsam. Opaque objects may be 

 fixed upon a piece of black paper gummed to the slide, or mounted on flat disks of 

 card or cork. They may also be placed in shallow cells resembling thin pill- boxes. 

 In all cases opaque objects should be placed on a black ground. 



All objects intended for microscopical observation should be protected by a cover 

 of thin glass, in order to prevent the entrance of dust and to exclude the air. The 

 fluid also in which many objects are placed for examination would rise in vapor, 

 which would condense upon the object-glass and occasion great inconvenience if 

 it were not prevented from evaporating by a thin glass cover. The glass cover 

 should not press upon the object, lest it should impair its distinctness or destroy 

 its structure. 



This evil is avoided by placing some substance, slightly thicker than the object, 

 around it and between the glasses, thus forming a little cavity for the specimen, 

 which may then be covered with thin glass without risk from, pressure. This 

 cavity is called a cell. 



