47 MAGNIFICA TION AND DRA WING. 



In comparing the methods of micrometry with the compound microscope, given 

 above {\\ 109, no, in, 116) the one given in \ 109 is impracticable, that given in \ 

 no is open to the objection that two standards are required, — the stage microme- 

 ter, and the steel rule; it is open to the further objection that several different 

 operations are necessary, each operation adding to the probability of error. Theo- 

 retically the method given in \ in is good, but it is open to the very serious ob- 

 jection in practice that it requires so many operations which are especially liable to 

 introduce errors. The method that experience has found most safe and expedi- 

 tious, afid applicable to all objects, is the method with the ocular micrometer. If 

 the valuation of the ocular micrometer has been accurately determined, then the 

 only difficulty is in deciding on the exact limits of the object to be measured and 

 so arranging the ocular micrometer that these limits are inclosed by some divisions 

 of the micrometer. Where the object is not exactly included by whole spaces on 

 the ocular micrometer, the chance of error comes in, in estimating just how far 

 into a space the object reaches on the side not in contact with one of the microm- 

 eter lines. If the ocular micrometer has some quite narrow spaces, and others 

 considerably larger, one can nearly always manage to exactly include the object 

 by some two lines. 



For those especially interested in micrometry, as in its relation to medical juris- 

 prudence, the following references are recommended. These articles consider the 

 problem in a scientific as well as a practical spirit : The papers of Prof. Wm. A. 

 Rogers on micrometers and micrometry in the Amer. Quar. Micr. Jour., Vol. I, 

 pp. 97, 20S ; Proceedings Amer. Soc. Microscopists, 1882, 1S83, 1887. Dr. M. D. 

 Ewell, Proc. Amer. Soc. Micrs., 1890 ; The Microscope, 1S89, pp. 43-45 ; North 

 Amer. Pract, 1890, pp. 97, 173. Dr. J. J. Woodward, Amer Jour, of the Med. Sci., 

 1875. M. C. White, Article Blood-stains, Ref. Hand-Book, Med. Sciences, 1885. 

 For the change in magnification due to a change in the adjustment of adjustable 

 objectives, see Jour. Roy. Micr. Soc, 1880, p. 702 ; Amer. Monthly Micr. Jour., 

 1880, p. 67. 



DRAWING WITH THE MICROSCOPE. 



\ 118. Microscopic objects may be drawn free-hand directly from the micro- 

 scope, but in this way a picture giving only the general appearance and relations 

 of parts is obtained. For pictures which shall have all the parts of the object in 

 true proportions and relations, it is necessary to obtain an exact outline of the 

 image of the object, and to locate in this outline all the principal details of struc- 

 ture. It is then possible to complete the picture free hand from the appearance 

 of the object under microscope. The appliance, used in obtaining outlines, etc., of 

 the microscopic image is known as a camera lucida. 



\ 119. Camera Lucida. — This is an optical apparatus for enabling one to see ob- 

 jects in greatly different situations, as if in one field of vision, and with the same 

 eye. In other words, it is an optical device for superimposing or combining two 

 fields of view in one eye. 



As applied to the microscope, it causes the magnified virtual image of the ob- 

 ject under the microscope to appear as if projected upon the table or drawing 

 board, where it is visible with the drawing paper, pencils, dividers, etc., by the 

 same eye, and in the same field of vision. The microscopic image appears like a 

 picture on the drawing paper. This is accomplished in two distinct ways : 



(A) By a camera lucida reflecting the rays from the microscope so that their di- 

 rection when they reach the eye coincides with that of the rays from the drawing 



