CH. VlH SCALE OF DRAWINGS 171 



appear with the image in the negative or in the tracing and the en- 

 largement or reduction can be found as above. Suppose the image 

 of the 10 cm. scale on the negative or in the tracing is 2 cm. long, 

 obviously the picture must be 2 cm. -r- 10 = -?$ or y, that is, the 

 picture is only one-fifth the size of the object. 



For any form of projection apparatus (fig. 109-114), the magic 

 lantern or projection microscope, after the image is traced, the object 

 is removed and a micrometer in half millimeters for the magic lantern 

 and low powers of the microscope is put in place of the object and 

 the image of the scale projected upon the drawing paper. Suppose 

 the image of one of the micrometer half millimeter spaces measures 

 15 millimeters, then the scale of the drawing 

 must be 30 (i.e., 15 -5- ^ = 30). 



If one is drawing from the projected image 

 of a negative or lantern slide it is necessary iM mm- 



to know the scale at which the negative or FIG. 103. MAGNIFIED 

 slide was made as well as the scale at which o? 



the drawing from the projected negative or INDICATING THE SCALE 

 i-j * i T- i AI_ i AT WHICH A DRAWING 



slide is being made, r or example, it the scale WAS 



of the negative is 50 times the size of the ob- 



ject, and the drawing is 10 times the size of the negative, the final 



drawing must be 10 X 50 = 500 times the size of the original object. 



If on the other hand the negative is yV the size of the original 

 object and the drawing is 5 times the size of the negative, the final 

 drawing will be the size of the negative (iV the original) multiplied 

 by the magnification (in this case 5) which is ro X 5 = -fy or y. 

 That is, the drawing is one-half the size of the original object. 



For the projection microscope with powers from 40 to 16 mm. a 

 micrometer in y mm. is good. For powers above 16 mm. it is better 

 to use a micrometer in o.i mm. and o.oi mm. (fig. 80). 



After the drawing has been made, remove the specimen and put 

 the micrometer under the microscope and draw a few spaces of the 

 micrometer image (fig. 103) giving the actual value of the spaces; 

 then one can compute the enlargement of the drawing by measuring 

 the image spaces and dividing by the actual value. For example, 

 suppose the image of one of the o.i mm. spaces measures on the 



