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XXIII. 



ON SHUTTEES FOE USE IN STELLAE PHOTOGEAPHY. By 

 J. JOLY, M.A., B.E., D.Sc, an Assistant to the Erasmus Smith 

 Professor of Experimental Physics, Trinity College, Dublin. 



[Read November 18, 1891.] 



Mr. a. Eambaut having in the course of conversation men- 

 tioned to me certain difficulties attending the construction of 

 shutters for stellar work with telescopes, I submitted to his judg- 

 ment and experience some designs which received revision at his 

 hands. These, as well as some yet untried notions of my own, are 

 embodied in the following brief Paper. 



The problem of constructing a shutter enabling one or more 

 particular stars in the field of the telescope to be covered at will, 

 or exposed for any fraction of the total exposure of the entire field, 

 may be dealt with according as the conditions are such that : — 



(a) One star only is to be eclipsed, which may be maintained 

 in the centre of the field, or 



{b) More stars than one are to be eclipsed. This condition 

 including that in which one star only is to be eclipsed, but in a 

 part of the field varying with the circumstances of each case. 



The first conditions are, perhaps, on the whole best met in the 

 following construction (see fig. 1) which is, in great part, due to 

 Mr. Eambaut. Here the shutter is carried at the extremity of a 

 fine steel needle projecting into the centre of the field. The 

 needle carries at its other extremity a small permanent magnet, 

 which is encircled by a coil of insulated wire. The needle being 

 freely pivoted, a current in the coil will deflect the magnet, bring- 

 ing it to point along the axis of the coil. The magnet, of 

 course, rotates through 180° with a reversal of the current. Thus, 

 it is evident, that by affixing the little shutter to the needle, so 

 that its plane is at right angles to the length of the magnet, 

 we may expose or cover, by switching a current, first in one and 

 then in the other direction, through the coil. If not sufficiently 

 dead beat, stops may be provided to bring up the magnet at 



