CH. X] MAGNIFICATION OF DRAWINGS 353 



For slight variations in size the change in distance is by far the 

 best and easiest change to make. 



If one has a drawing table (fig. 182) it is very simple to push it 

 farther from or closer to the projection apparatus. 



If the drawing shelf is used it can be raised or lowered (fig. 183). 



If the simple apparatus is used on an ordinary table the entire 

 microscope can be raised for higher magnifications or for lower 

 magnifications the drawing surface can be raised to bring it closer 

 to the microscope tube (fig. 193-204), or the microscope can be 

 lowered on its adjustable support (fig. 198). 



DETERMINATION OF THE MAGNIFICATION OF A DRAWING 



508. For getting the magnification it is necessary to use for 

 an object a transparent micrometer with known divisions upon it. 

 For most of the work done a micrometer with heavy lines every 

 half millimeter is satisfactory. These lines may be ruled on glass 

 and filled with graphite, or they may be made by photography 

 (see 5o8a). 



For example, if the micrometer used has half millimeter spaces, 

 the image projected on the screen will be magnified more or less 

 according to the distance of the screen and the objective used. 

 The exact size of the image is easily measured on the drawing sur- 

 face with a millimeter scale. Suppose that two of the half milli- 

 meter spaces were used as object, the object would be one milli- 

 meter in actual size. If the image of two spaces projected on the 

 screen or drawing surface measured 2 5 millimeters then the magni- 

 fication would be 25. 



508a. One can make a satisfactory micrometer for determining the 

 magnification of drawings as follows : Make a negative of the millimeter scale 

 (fig. 178, 21 1) making the picture exactly half the size of the original scale, then 

 the spaces will be half millimeters. As the scale is black with light lines the 

 negative will show dark lines with intervening clear spaces exactly as is a glass 

 micrometer. 



If so desired the micrometer lines may be covered with Canada balsam and 

 a cover-glass applied as for microscopic specimens. (See The Microscope, 

 354~5> P- 2 57)- This would protect the lines and make the specimen more 

 transparent. A lantern-slide plate is the best for making the negative, as it 

 gives transparent lines. 



