Ch. XI] MODELS FROM SERIAL SECTIONS 417 



or the projection apparatus method (§ 293) is good. With the per- 

 fecting of projection apparatus that method is far the best because 

 one can sit in a comfortable position and use both eyes. It is indeed 

 as simple as tracing the outline of actual pictures. 



By making negative prints directly on one of the developing papers 

 (§ 363) , drawing for models may be wholly avoided. 



§ 673. Avoidance of distortion and of inversion. — In the drawings 

 for models one must of course avoid all distortion (§269) and the 

 inversion of the image (§277). Both these defects are easily avoided 

 if one keeps in mind the optical principles involved, and follows the 

 directions given in Ch. VI. 



§ 674. Use of the 6-volt, concentrated filament lamp as a source 

 of light. — From the experience of the author nothing equals the 

 direct-current arc light for all exacting work in drawing and projec- 

 tion, and for the dark-ground illuminator, but the care required to 

 keep the arc lamp going and to keep the crater centered is so great 

 that the less brilliant light from the 6-volt lamp which requires abso- 

 lutely no adjustment after being once properly arranged is very 

 acceptable (§ 362). The 6-volt lamp with a transformer is used only 

 on an alternating circuit. As most lighting circuits are now alternat- 

 ing it is a great advantage; and as this lamp with its transformer 

 can be used anywhere wherever there is an ordinary electric light 

 socket, it is exceedingly convenient. If it is to be used on a direct 

 current circuit no transformer is used, but the current must be drawn 

 from a storage battery, not from a no or a 220 volt circuit from a 

 dynamo. 



§ 675. Connections of the transformer. — If alternating current 

 and a transformer are used, the transformer must be connected to 

 the supply by means of the small connecting wires. The connection 

 with the lamp is by the large terminal wires. Ordinarily the terminals 

 of the transformer are marked so that no mistake need be made. 

 Theoretically the transformer does not modify the energy; it either 

 raises or lowers the voltage or pressure. For the purposes here used 

 the transformer lowers the voltage, and is called a "step down trans- 

 former." As the activity or wattage of which the current is capable 

 is not changed by the transformer, and as the wattage is the voltage 



