BY A. B. BIGGS. 99 



A very useful dark-field Micrometer, embracing the greater 

 portion of the field, is the Bar Micrometer. Mj own form 

 of it is a modification of that used by Lacaille in the prepara- 

 tion of his valuable Catalogue of Southern Stars. His was a 

 rhomboid cut out of a piece of thin brass and placed in the 

 focus of the eye-piece ; mine is an equilateral triangle, formed 

 of watch hair-spring. The differences of right ascension and 

 declination are obtained by timing the passages in and out of 

 the triangle. It is a very useful instrument for faint objects 

 which will not bear illumination of the field, and especially 

 for comet work. 



The Micrometer, par excellence, for general work is doubt- 

 less the Filar Position Micrometer. A description of this is 

 of course superfluous to those at all acquainted with telescopic 

 work. The measurement is effected by parallel spider lines, 

 moved to and fro by fine screws, the measu es being read off 

 by the number of turns, and by graduations on the screw 

 heads. The scale is revolved by a pinion and wheel, so as to 

 make a cross spider line intersect the objects to be measured, 

 and the position angle is read from a graduated circle. This 

 instrument is specially convenient for differences of declin- 

 ation ; but for direct oblique distances, is difficult to use with- 

 out a steady driving clock for the telescope. It is a delicate 

 and expensive aparatus. 



Many other forms and methods of Micrometer measurement 

 are adopted, which it will be unnecessary to further refer to. 

 I will now go on to describe my own, first giving the general 

 principle. 



If a strip of glass (A), coated with black paint, and having 

 two fine converging lines cut through the paint, at an angle 

 of 10 or 15 deg., be placed face to face with another piece 

 of glass (B), similarly coated, and having a single line ruled 

 across it — this line being placed so as to cross the lines of A 

 — the intersection of the lines vsdll show as luminous points 

 by transmitted light. On sliding the slip A along, these 

 points will recede or approach until they coalesce at the 

 point of the angle. Now, if an image of these points can be 

 projected into the field of the telescope, and brought into 

 juxtaposition with the pair of objects whose angular distance 

 is to be measured, we obviously have the means, by a proper 

 adjustment of the points as to distance and parallelism, of 

 determining the measurement required. The position of the 

 slide A is read upon a graduated scale, the value of which 

 is determined by well-known astronomical methods. 



The projection of the image into the telescope is effected 

 by means of an adjustable camera-lucida, constructed from 

 a selected micro, cover-glass and attached to the eye-piece. The 

 whole carrying arrangement of the glass plates is made to 



