Permanence of the Eutherfurcl Photographic Plates. 19t 



'makers for Prof. Bakhuyzen, and which has been fully described 

 by him (Bulletin du Comite Permanent, Tome I., p. 169). A very 

 brief description of the instrument n\a.y perhaps not be out of 

 place here. It consists essentially of an arrangement for com- 

 paring distances on the plate with a carefully graduated metal 

 scale. To accomplish this a micrometer microscope is mounted 

 on trunnions in such a way that by revolving the microscope 

 through a very small angle it is possible to direct it towards either 

 the plate or the scale. In this way it is possible to make successive 

 bisections of the star and scale with the same microscope. Thus 

 the position of the star with respect to the scale is measured 

 micrometrically. The trunnions are parallel to the scale. The 

 plate itself is mounted on a very firm casting which slides upon 

 ways that are at right angles to the scale. The microscope 

 with its trunnions slides upon another pair of ways that are 

 parallel to the scale. Thus it is possible to center any star on 

 the plate under the microscope by moving the microscope along 

 its slide parallel to the scale, and the plate on its casting along 

 the ways at right angles to the scale. The star having been 

 centered in this way, a comparison with the scale gives us one 

 coordinate of the star on the plate. It is evident from this 

 that the coordinate as measured will be influenced by any diver- 

 gence from exact straightness that may exist in the ways 

 carrying the plate and its holder. For this reason, the greatest 

 care has been taken to make these ways straight. The guiding 

 way is a cylinder, because a cylinder can be constructed more 

 nearly exact than any other geometrical form. The other way is 

 merely a plane, and acts only as a support, not at all as a guide. 

 In order to measure the second coordinate, it is necessary to turn 

 the plate through a right angle, and then to proceed as before. To 

 accomplish this a graduated circle is provided, read by two mi- 

 crometer microscopes to single seconds. 



The microscope used throughout the present series of observa- 

 tions for bisecting the plate and scale is provided with two dis- 

 tinct sets of wires, moved by separate screws at right angles to 

 each other. Both sets of wires are in focus at the same time, 

 though, of course, the two sets are not exactly in the focal plane 

 of the microscope. The object of having two sets of wires is to 

 measure plates that have had a reseau, or network of straight 

 lines, photographed on them. For such plates, this double 



Annals N. Y. Acad. Sci., IX, October, 1896.— 14. 



