44 III. PROTOPLASMIC MOVEMENTS MICRO. DRAWINGS. 



therefore the simplest and best for measuring the size of a micro- 

 scopical object. If, that is, we have attained the necessary 

 accuracy in drawing, in order to reproduce even slight variations 

 in size with fidelity, and if we know the definite enlargement of 

 the object which we have drawn at exactly the same distance, 

 it needs only to divide the size of the drawing by the known 

 enlargement to get the actual size of the object. If, e.g., one cell 

 of the hair of Tradescantia appears, with 240 times enlargement 

 of its figure, to be 9 mm. broad, this indicates an actual breadth 

 of ^ mm., i.e., of 0-0375 mm. This method gives in the 

 simplest way so accurate a result, that in our studies we can 

 confine ourselves to it. 



Various other contrivances have been introduced for the 

 purpose of aids to drawing. Some of these, like the Wollaston 

 Camera Lucida, require the body of the instrument to be placed 

 horizontally, and the instrument as a whole to be raised on a 

 pedestal. This can of course only be used with instruments 

 which admit of this position ; and for working purposes it is, 

 besides, objectionable in several ways. A very cheap form for 

 use thus is Dr. Beale's neutral-tint reflector, which fixes on the 

 eye-piece, making with its glass an angle of 45. The student, 

 when he chooses a camera or drawing-prism, should, however, 

 always select one for use with the instrument in the vertical 

 position ; and, as he may not improbably obtain one from a 

 maker who is not the maker of his instrument, he should always 

 send the eye-piece of the latter, so that the fittings of the camera 

 may be adjusted to the size of this. Zeiss's camera is adjusted for 

 eye-pieces of the continental size, a size much used now by Eng- 

 lish makers for their smaller instruments. Of whatever camera 

 is chosen, the method of adjustment upon the eye-piece must be . 

 learned from the maker's description (though usually very easy 

 to find out for one's self) ; the rules laid down above for learning 

 how to draw are equally applicable to all of them. Lastly, the 

 quality of the drawing depends on two factors : the accuracy of 

 the observer, and the skill of the draughtsman. Endeavour 

 to draw accurately what is, and do not let your observation be 

 corrupted by forming impressions, or by placing some interpreta- 

 tion upon what you see. If you do this latter, your drawing 

 serves only to illustrate an interpretation which may prove to 

 be wholly inaccurate ; if you do the former, your drawing should 

 be consistent with any interpretation. Published figures, particu- 



