198 Scientific Proceedings, Royal Dublin Society. 



replaced by a small hair-spring controlling the position of the 

 shutter when no current is passing. "We may meet the conditions 

 {a) by another method if through any reason the accommodation of 

 the coil to one side be attended with inconvenience. But this will 

 appear in treating of shutters to meet condition {h). In this case, 

 by arranging the foregoing contrivance (fig. 1) so that the needle 

 can be advanced more or less into the field by moving it through 

 a hollow sprung spindle carrying the magnet, and by arranging 

 either to rotate a ring encircling the field carrying with it the coil, 

 or to clamp this at any point on the edge of a fixed ring, any part 

 of the field can evidently be reached by the shutter; and so any one 

 star in any part of the field eclipsed. Freedom to move the coil in a 

 circular direction round the field is, probably, in any case advisable 

 with this form of shutter in order that the needle may be brought 

 along a radius in which it would not itself eclipse lesser stars. 



It is, of course, possible to set the needle of the shutter, when 

 thus provided with a circular motion round the field, so that by 

 fixing a second shutter upon the needle two stars lying upon a 

 radius, or nearly so, might be eclipsed. But the problem of 

 dealing with more than one star, with this form of shutter, is best 

 solved by providing more than one coil. Thus, by fitting two 

 coils on the edge of the field, an easy adjustment of the needles 

 in each enables any two stars to be eclipsed. The same current 

 may operate each shutter, or the exposures of each may be regulated 

 at will. 



The next pattern of shutter depends on the same principles, 

 but the coil, instead of being small and to one side of the field, 

 surrounds the whole field of the photographic plate. The coil is 

 wound of very fine wire, and is only some five millimetres in 

 length {i. e. axially). Its width may be six or eight millimetres. 

 A current through this coil produces a magnetic force all over the 

 field, and suitably pivoted magnets anywhere in the field will set 

 pointing along the axis of the coil in polar directions, depending 

 upon the direction of the current. It is now only necessary to 

 provide a simple and ready method of adjusting such magnets, 

 each carrying its shutter, or itself serving as shutter, in any part 

 of the field. 



To meet conditions [a) with this form of shutter, we arrange a 

 single projecting needle, fixed in position, and extending from the 



