4 6 



PRACTICAL HISTOLOGY. 



place of a prism. It may be fitted to a support that can be clasped to 

 the eye-piece. A cover-glass answers the purpose. Dr. Beale, how- 

 ever, recommended that the glass should be of neutral tint, to diminish 

 the glare from the paper, an advantage certainly. This is, doubtless, 

 the cheapest apparatus for drawing, but it is certainly not so good as 

 the prism ; it cannot be so good, because in the prism the rays from 

 the object undergo total reflection, whereas they are only partially re- 

 flected from the surface of the plate ; moreover, the microscope must 

 be inclined. 



FIG. 39. Drawing with parallel plate reflector, 



Success in using a, b, or c will not, however, be attained 

 unless the following be carefully attended to. The object 

 must not be too brightly illuminated ; a black pasteboard 

 shade should be placed between the window and the eye to 

 cut off extraneous light, and the paper on which the image 

 appears to lie should be rather dimly lighted, otherwise the 

 point of the pencil will not be clearly seen. The beginner 

 should always dose one eye. 



d. Squared glass. A plate of glass with fine squared 

 lines (Fig. 43) placed in the eye-piece is of much service for 

 enabling one to draw objects of their exact size and form 

 without a camera. The same piece of glass should be 

 engraved, so as to serve as an eye-piece micrometer. 



59. Estimation of the Magnifying Power of the 

 Microscope. A stage micrometer is necessary for the 

 determination of the magnifying power of any combination 



