3G4 DENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



of the field of view be not well illuminated, the prism 

 may be adjusted by turning on its pivots, or a slight move- 

 ment round the eye-piece, the observer will now bring 

 his eye so near the edge of the prism, that he may be able 

 to see at one and the same time, the image of the object, 

 and the point of a pencil. When he has accomplished 

 this, the pencil may be moved along the outline of the 

 image so as to trace it on the paper ; however easy this 

 may appear in description, it will be found somewhat 

 difficult in practice, and the observer should not be foiled 

 in his first attempts, but persevere until he accomplishes 

 his purposes; he may find that he can see the pencil-point, 

 and all at once it disappears ; this happens from the move- 

 ment of the axis of the eye. The pencil should be kept 

 on the paper, and the eye moved about until the pencil is 

 seen again, when the eye is to be kept steadily fixed in 

 the same position, until the entire outline is traced. The 

 operation may be conducted by lamplight, or the diffused 

 light of day. If, after the drawing is made, the object on 

 the stage be removed, and the micrometer fixed thereon 

 in its place, and an image of its magnified divisions drawn, 

 which being compared with that made of the magnified 

 object, the dimensions of the latter are readily ascertained. 

 The size of the picture drawn on the paper, will like all 

 others made by the camera, depend on the relative dis- 

 tance between the object and the paper. 



Polarizing Apparatus. For the purposes of polarizing 

 light, various substances have been employed ; sometimes 

 a crystalline mineral, called tourmaline, in the form of a 

 thin plate, confined between-two thin glasses, and set in a 

 brass cell ; but the most useful for the microscope is a 

 crystal of Iceland spar, called Nicol's prism, consisting of 

 a romb of the spar divided into two equal portions, in a 

 plane passing through the acute lateral angles, and nearly 

 touching the obtuse solid angles, the cut surfaces havingbeen 

 carefully polished, are then cemented together with Canada 

 balsam, so as to form a romb of nearly the same shape as 

 it was before cutting, and capable of transmitting a single 

 image only. As adapted to the microscope, one of these 

 prisms is fitted in a brass cell by means of a piece of cork, 

 so constructed that it may be placed in the circular open- 

 ing in the frame, H, beneath the stage-plate, and is moved 

 up and down by the lever, I ; this lower prism is called 



