BY A. B. BIGGS. 101 



any irregularity in the figure ; I think more so than with 

 respect to coincidence with spider lines, as in the use of the 

 filar micrometer, especially when, without a driving-clock, the 

 object is moving obliquely across the field, and only a 

 momentary contact can be obtained in passing. The similarity 

 of the images in the former case favours the comparison. 

 Fig. 3 shows the general arrangement of the appai-atus 

 as applied to a Newtonian reflector. 



My first experimental arrangement was fitted to my 3in. 

 refractor, and was a very primitive affair, the carrier being of 

 tin, revolving in a paper tube. For a refractor, a different 

 arrangement from that described above has to be adopted. 

 With the Newtonian reflector, the position of the scale being 

 at right angles with the direction of- vision, a single reflection 

 at 45deg. throws the image into the eye-piece. With the 

 refractor, on the other hand, the only practicable position for 

 the apparatus is on the body of the tube towards the object- 

 glass ; that is in the direction of vision. This necessitates 

 an intermediate reflection at an angle of 45, to throw down 

 the image of the scale upon the camera-lucida. (Fig. 4.) 



The apparatus admits of very considerable elaboration and 

 development; as, for instance, star photometi'y. Further; 

 the whole apparatus may be made to travel to or from the eye 

 on a suitable slide, having a graduated scale ; a single plate 

 with parallel lines being placed in the plate-holder. By this 

 arrangement planetary discs and differences of declination 

 may be read off, as with the filar micrometer. I will not, 

 however, add to the tediousness of this paper by further 

 reference to this matter. 



I must, in closing, express my obligation to Mr. Alex, 

 Wallace, of this city, a clever amateur mechanic, for his 

 kindness and generosity in the successful construction of my 

 present apparatus. 



