300 TECHNICAL MICKOSCOPY. 



since the scale is based on the assumption that the diameter of the 

 object is the unit. When the image is smaller than the object the 

 scale must of course take the form of a fraction. 



The drawing obtained by means of the camera lucida must be 

 carefully compared with the microscopic image, and corrected 

 before the details are filled in. It may easily happen that import- 

 ant dimensions for instance, the thickness of the walls in cellular 

 tissues or that certain peculiarities of form may appear somewhat 

 inaccurate in the drawing, and that the aid of the free hand is 

 absolutely requisite for perfect reproduction of the outlines. The 

 drawing must therefore be completed with the free hand ; this 

 part of the task is the most difficult, for experience and skill are 

 required on the part of the draughtsman. In difficult cases the 

 unpractised hand cannot produce even an approximately true 

 image; it is hence strongly recommended that all intending 

 microscopists should learn from the very commencement to 

 delineate accurately what they see. Books give but slight assist- 

 ance in this matter ; a teacher or constant practice will be of far 

 more service. 



The character of the drawings should be variously modified 

 according to the purpose in view. In many cases a mere outline 

 drawing showing the morphological or anatomical proportions is 

 quite sufficient ; in others, it is necessary to add a purely conven- 

 tional shading of certain parts of the drawing, to which special 

 attention is directed. Similarly, with stratified or fibrous substances 

 the course of the strata or fibres may be represented by straight 

 lines ; it must then be remembered that the lines do not represent 

 the thickness or fineness of the strata, but merely indicate their 

 presence and illustrate their course. 



The task is very different if the drawing is to be true to nature 

 in the smallest details visible that is to say, as far as the distri- 

 bution of light and shade may reasonably be represented. In this 

 case we have choice of two methods of procedure. The nearest 

 approach to nature would of course be made by exact reproduc- 

 tion of the gradations of light as they really appear in the micro- 

 scopic image ; the field of view itself would thus receive a tint 

 corresponding to its brightness. The denser layers of a membrane 

 would therefore be represented as lightest, the more open layers 

 somewhat darker, and the shadows at all points of transition 

 approaching a deep black. In this way a skilled hand can so far 



