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Scientific Proceedings, Royal Dublin Society. 



brouglit up at the back of the plate suggests itself ; the 

 magnets and shutters being located as before. 



Lastly, we may use permanent magnets in this pattern as in 

 the other pattern. Of course we are then compelled to arrange all 

 the magnets parallel to a common line joining the poles of the 



Fig. 3. 

 permanent field. The magnets themselves may then constitute 

 the shutters, the star being covered by one of the poles when no 

 current is passing in the coil. A square frame to the field is most 

 convenient with this construction, enabling the supports of the 

 magnets to be slid up and down in parallel directions. The 

 pivoting of the magnets may be effected much as before. 



In the discussion, Mr. Eambaut exhibited plates exemplifying 

 the use of a shutter in securing greater definition of a bright star 

 exposed for only a fraction of the total time of exposure. He 

 considered the principal use of these shutters would be found in 

 parallax work, when a bright star could be maintained in the 

 centre of the field. He used, preferably, a coil and magnet 

 placed to one side of the field, a projecting wire carrying the 

 shutter, and found it well to use stops to check the oscillations 

 of the needle. The action of this was very prompt. A pendulum 

 made contact at the centre of its swing for a fractional part of the 

 whole period of its oscillation. 



