CH. X] DRAWINGS FOR MODELS 357 



the object, or beside it. It will be at the same scale of magnifica- 

 tion or reduction as the drawing. 



In practice some lines of the image of the scale are made beside 

 the drawing. For example, suppose the image of one centimeter 

 measured on the drawing was 10 centimeters long, one would know 

 that the drawing is 10 times larger than the object. If the length 

 of the centimeter on the drawing was only one-half centimeter long, 

 then one would know that the drawing is only half as large as the 

 object and so on ( 5o8a, 5ioa). 



DRAWINGS FOR MODELS 



511. Drawings for models. These are made much more 

 easily with projection apparatus than with the camera lucida or in 

 any other way. The simple drawing outfit for use on the house 

 circuit described above makes it possible for every laboratory and 

 indeed every private worker to use this effective method, even if 

 complete projection apparatus and heavy lantern currents are not 

 available. 



In making drawings for models several steps must be taken in 

 order that the resulting model shall be anything like a true repre- 

 sentation of the actual object. 



(1) The object (embryo, etc.) should be photographed at a 

 known magnification before it is sectioned. 



(2) The sections should be made of a known thickness 

 1 5 [A, etc.). 



510a. The general law for magnification and reduction. With a given 

 object the size of the image depends directly upon the relative distance of the 

 object and of the image from the center of the lens (fig. 185, 209, 210). If the 

 image is farther from the center of the lens than the object then the image will 

 be larger than the object; conversely if the image is nearer the center of the 

 lens than the object then it will be smaller than the object. 



For example, if the image is to be ten times as large as the object the image 

 must be ten times as far from the center of the lens as the object. 



Conversely, if the image is to be one-tenth as large as the object it must be 

 formed only one-tenth as far from the lens as the object. 



In lantern-slide and micro-projection, and in photo-micrography the image 

 is much larger than the object and correspondingly more distant from the cen- 

 ter of the lens. In ordinary portrait photography and in landscape photo- 

 graphy the image is much smaller than the object, and consequently the image 

 is much nearer the lens than the object (see also 392a). 



