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 service- 

 able, but one must always keep in mind that the dimmer the light- 

 source, 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 projec- 

 tion (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 

 (sec also 485). If low amperages arc 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 arc 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 



