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328 RADIANTS FOR DRAWING [Cn. X
RADIANTS FOR DRAWING APPARATUS
§ 461. General. — The best light for projection is naturally the best light for drawing with projection apparatus. One must always keep in mind that a rather dim light in a perfectly dark room, after one has been long enough in it to acquire twilight vision, may seem quite brilliant. The old observers with their very dim artificial lights understood this well, and did much with projection apparatus which at first sight would seem impossible to us.
The electric arc and other brilliant artificial lights are so common at the present that many have come to feel that they cannot see at all unless the object is flooded with light. But, excepting those who are night-blind, that is, have poor twilight vision, much can be done with the Welsbach mantle light, the alco-radiant mantle light, etc. Even a kerosene lamp of good quality is very serviceable, but one must always keep in mind that the dimmer the lightsource, the darker must be the work-room, and the more care must be taken to avoid stray light. Too high powers should not be used with weak lights. For high power drawing very brilliant light is necessary.
§ 462. Arc lamp with direct current. — This is, of all the artificial sources, the most satisfactory for drawing, as for projection (fig. 3). With it the drawing room need not be very dark, and one can obtain sufficient light for the highest powers with which it is desirable to draw. Ordinarily a 5-10 ampere current is sufficient (see also § 485) . If low amperages are used the apparatus is not so greatly heated as with higher amperages, and furthermore the specimens are less liable to injury from overheating.
The same lamp that is used for projection is suitable for drawing. There is some advantage in having an automatic arc lamp, then the artist will not have to bother about the lamp except to supply it with proper carbons, and to see that they are in proper position. With the hand-feed arc lamps the carbons must be brought closer together about every 3-5 minutes. It is a convenience if the artist