APPENDIX. — DRA.WING THE OBJECT. 217 



object, and then transferring its dimensions to paper. Knowing the magnifying 

 power of the combination used and the apparent dimensions of the object, the 

 actual dimensions of the latter may readily be determined. 



In practice, however, it is much more convenient to read off the dimen- 

 sions of the object by means of a micrometer scale, placed in the eye-piece. 

 For this purpose detach the micrometer from the glass slide and insert it in the 

 eye-piece in such a manner that it will rest upon the diaphragm between the two 

 lenses. If necessary, screw out the eye-lens of the eye-piece a little, until the 

 lines are accurately in focus. Let, now, any object whose dimensions have pre- 

 viously been measured with accuracy by one of the methods already described, 

 be placed upon the stage and accurately focussed. If another accurately ruled 

 micrometer is available as an object, so much the better. The lines of the eye- 

 piece micrometer may now be seen superposed over the object on the stage, 

 and the number of divisions of the former corresponding to the length of the 

 object may be seen. Suppose, for example, the objective is known to be exactly 

 one-five-hundredth of an inch in length, and that it subtends five of the small 

 spaces of the eye-piece micrometer, each space, therefore, will have a value, 

 with this optical combination and with this length of tube, of one fifth of one- 

 five-hundredth, or one twenty-five-hundredth of an inch. In like manner, the 

 value of the spaces in the eye-piece scale may be determined for all the differ- 

 ent combinations and for different tube-lengths. 



The results should be placed in tabular form for easy reference and for 

 habitual use. 



DRAWING THE OBJECT. 



No part of the student's work in vegetable histology has a higher disciplin- 

 ary value than drawing the structures he sees under the microscope. The effort 

 to represent a structure faithfully leads to close and accurate observation, one 

 of the most important qualifications of a scientist. The student who practices 

 it will see more with his microscope than one who does not, and he will also 

 reach sounder conclusions regarding what he sees. Moreover, good drawings 

 are a very necessary means of recording the results of observations if they are 

 to have any permanent va!ue. No record of microscopic work is complete 

 without them. By mere verbal discription, however accurate, it is scarcely 

 possible to convey to another a clear and vivid idea of a complicated microscopic 

 structure. Nor can photography, useful as it is, take the place of the draughts- 

 man's art. Success in conveying an accurate idea of a fact of structure often 

 depends nearly as much upon leaving out unnecessary and confusing details, as 

 upon presenting the essential things. But a photograph cannot discriminate. 

 Moreover, a photograph represents clearly only what lies in a single visual plane. 

 That which lies either above or below it is more or less indistinct, confused and 

 confusing. Photography may and will perform an important part in microscopic 

 research ; it may oftentimes very materially aid the draughtsman's art, but it 

 can never take the place of it. 



Very good and accurate drawings of microscopic objects may be made 

 without the use of the camera lucida, an instrument which is capable at times 





