CH. X] LIGHT SHIELD FOR DRAWING 351 



SHIELDING THE DRAWING SURFACE FROM STRAY LIGHT 



505. Shield for working in a dark room. If one works in a 

 dark room all that is necessary to screen the drawing from stray 

 light from the arc lamp, when the lamp is at right angles to the 

 microscope, is a blackened cardboard shield (fig. 193). 



If the lamp is in line with the microscope, it will be necessary to 

 put a shield with a perforation for the light beam either before the 

 beam reaches the microscope, or it may be put over the tube of the 

 microscope so that it will shield the drawing surface. 



506. Drawing in a light room. If this is necessary one should 

 get in as shaded a part of the room as possible. To screen the 

 drawing surface there are two ways : 



(1) There may be a cloth for enclosing the drawing surface and 

 the head of the artist. This is like the plan used in focusing a 

 photographic camera (fig. 204). 



(2) By means of cardboard, or of a wire frame and cloth cur- 

 tains, a box or tent is built around the drawing surface enclosing 

 also the microscope tube. The end of the box next the draughts- 

 man is open sufficiently for him to see the image (fig. 196, 198). 

 The drawing surface should look toward some dark furniture 

 or a dark or shaded part of the room, and except for the most 

 exacting work the surface will be sufficiently shaded. For the most 

 exacting work, and for the greatest freedom from accessories, the 

 evening or a dark room in the daytime offers the best facilities 

 ( 453)- 



How TO GET ANY DESIRED MAGNIFICATION IN A DRAWING 



507. The magnification can be varied by any of the following 

 ways, or two or more of the ways may be combined. 



(a) By using a higher or lower objective. 



(b) By using an amplifier, of greater or less power, with the 

 objective. 



(c) By using a higher or lower ocular with the objective. 



(d) By changing the distance of the drawing surface; the 

 farther it is away in any given case the larger will be the image, and 

 the nearer it is the smaller the image ( 5ioa). 



