Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine (1914)

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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 curtains, a box or tent is built around the drawing surface enclosing also the microscope tube. The end of the box next the draughtsman 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).