170 SCALE OF DRAWINGS [Ch. VI 



In such an object all the different structures can be shown, and by 

 omitting some of the fibers the others may be made plainer with- 

 out undesirable enlargement of the entire figure. (5) If a drawing 

 of a given size is desired and it cannot be obtained by any 

 combination of oculars, objectives, and lengths of the tube of 

 the microscope, the distance between the camera lucida and the 

 table may be increased or diminished until the image is of the desired 

 size. This distance is easily changed by the use of a book or a block, 

 but more conveniently if one has a drawing board with adjustable 

 drawing surface like that shown in fig. 102. (6) It is of advantage 

 to have the camera lucida hinged so that the prism may be 

 turned off the ocular for a moment's glance at the preparation, 

 and then returned in place without the necessity of loosening 

 screws and readjusting the camera. This form is now made 

 by several opticians, and many of them add graduations so that 

 the angle of the mirror is readily seen. 



§ 276. Scale of drawings. — The scale should be given for every 

 drawing (fig. 103). Sometimes the drawing is larger than the object, 

 as with microscopic specimens, and sometimes it is of the same' size 

 or much smaller, as in drawing large objects. 



In getting the scale at which an object is drawn with the microscope 

 or projection microscope, the object is removed and a micrometer 

 in half millimeters (fig. 65) for low powers and one in tenths and 

 hundredths of a millimeter (fig. 80) for high powers is put in place 

 of the specimen. The image of the micrometer lines and spaces will 

 be of the same enlargement as the drawing, provided nothing has 

 been changed except the micrometer for the object. If now a few of 

 the lines of the micrometer image (fig. 80, 103) are traced at one 

 corner of the drawing paper and their actual value given, the enlarge- 

 ment can be determined accurately as follows: Suppose the mi- 

 crometer spaces are tenth millimeters, and the image of the spaces 

 measures 2 millimeters, the enlargement must be the size of the 

 image divided by the size of the object or 2 -f- 0.1 = 20, that is, the 

 image is 20 times the size of the object. 



In using the photographic camera for negatives or for tracing, 

 if the metric scale (fig. 104) is put with the object its image will 



