JoLY — On Shutters for use in Stellar Photography. 197 



the extremity of its swing. From the small mass of the moving 

 parts there is not much apprehension of vibration in this form 

 of shutter. It might he well to secure a dead heat arrange- 

 ment, not by the use of stops, but by attaching a disc of thin 

 paper or mica to the magnet. This disc might fill the space 

 within the coil, and the air-friction of it would be considerable. 

 Promptness, on the other hand, if a desideratum, would be sacrificed 

 by this arrangement. 



Fig. 1. 

 A variation of this arrangement is to place a curved permanent 

 magnet (this is shown in fig. 1) within the coil, so that its poles 

 normally control the position of the moving magnet till the attrac- 

 tive force is overcome by the superior magnetic force of a current 

 traversing the coil. The plane of the shutter is here arranged in 

 the plane of the moving magnet. In the first case a very brief 

 reversal will not operate the shutter unless the moving magnet in 

 its extreme position is inclined to the axis of the coil, the plane of 

 the shutter not being arranged at right angles to the length of 

 magnet. This will be readily understood on a little consideration. 

 Hence to operate the first form of shutter reversals should be made 

 at intervals say of one minute, and maintained for a second or 

 two, an operation conveniently effected by clockwork, or even by 

 hand. In the second case the moving magnet is deflected, not by 

 a reversal of current, but by completing circuit. A very prompt 

 deflection follows, as the needle is in a good position to feel the 

 electro-magnetic effect. The permanent magnet may again bo 



E2 



