410 OPTICAL PROJECTION 



good results. Lamps can, however, only be thus projected by 

 a lantern whose light can be readily and considerably lowered ; 

 for while a dim projection of the non-luminous parts very much 

 adds to the effect, any brightness in that part of the image 

 utterly spoils it. If the carbon-points themselves are too 

 brilliant, a small diaphragm may be placed over the objective, 

 and the light in the lantern increased accordingly. Another 

 advantage of this method is that the poles are projected sharply 

 and uncoloured, owing to the employment of the achromatic 

 lantern objective. Ordinarily the arc (in lantern) is projected 

 by the condenser alone, passing the rays through an aperture 

 small enough to tone down too great brilliance. 



With regard to the mechanical or motor effects of the 

 current, their elements have been already mentioned in 241. 

 If adequate range of current be at command, however, a 

 model made in such skeleton form as to show the essential 

 arrangements distinctly, may be projected in motion ; the fewer 

 skeleton coils producing slower motion, and so enabling the 

 action to be better followed. A model of a Gramme ring with 

 such open skeleton coils can thus be projected in action very 

 clearly, or can be reduced by a brake down to a visible rate if 

 required. 



Of the optical effects of the current, the rotary power has 

 been already described in the chapter upon polarised light. 

 Whether the luminous effects in vacuo can be projected, de- 

 pends chiefly upon their brilliance, the size of the lens, and 

 the darkness of the room ; the primary apparatus itself 

 needing to be carefully shielded from view. De la Kive's 

 rotating arc can be projected fairly well, if the apparatus is 

 in good order ; and it depends upon its size and that of 

 the audience whether this method, or that of direct vision, 

 is to be preferred. As already pointed out at the commence- 

 ment of Chapter XIV., considerations of this kind must in 

 many cases govern the choice of method ; and they apply 

 to electrical experiments with especial force. The more 



