JoLY — On Shutters for use in Stellar Photography. 



199 



coil nearly into the centre of the field. At its extremity a small 

 piece of magnetized watch-spring is carried pivoted in a little 

 stirrup. The magnet carries a projecting shutter, balanced hy a 

 small bob. Its motion is through an arc of about 120°, in its 

 extreme positions being brought up abruptly against a cross wire 

 attached in the arch of the stirrup. (See fig. 2, which shows, to 



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Fig. 2. 



a large scale, two elevations parallel to the axis of the coil.) A 

 brief reversal of the current will deflect this magnet, throwing it 

 over in the stirrup, so that the star becomes uncovered. The 

 extremely small mass of the moving parts, and the elasticity of 

 the needle, secures almost complete absence of vibration. 



To use this shutter we pass a current through the coil, so that 

 the shutter is brought parallel to the sensitive plate. It is then 

 some five or eight mms. from its surface. The telescope is now 

 moved till the bright star is eclipsed by the shutter. Reversal of 

 the current now secures exposure. All is ready then for putting 

 in the plate. To meet conditions [h) with this form, the needle 

 is arranged to move with sufficient stiffness through a small 

 spring clip, sliding upon the ring of the coil. The clip may thus 

 be adjusted to the most suitable point at the edge of the field, and 

 the needle more or less advanced along the radius. If more than 

 one star is to be covered, other clips and shutters are brought into 

 use, as shown in fig. 3. The exposures — in the case of more than 

 one shutter being used — are, of course, the same for all the 

 shutters. 



A variation of this form, in which the coil is separately 



