OF ARTS AND SCIENCES. 367 



double stars, of nebuloe, of red stars, or of objects having singular 

 spectra, as planetary nebulre, banded stars, and variables of long 

 period. Suppose that the field of view had a diameter of somewhat 

 over one minute of time, and that a small motor was attached to the 

 mirror which would move it uniformly over 5° in declination in this 

 time, and then bring it quickly back to its first position. Tiie observer 

 would then have presented to him a series of zones 5° long and one 

 minute wide. The sweeps should overlap by a small amount, so that 

 the entire region could be covered in a single evening. The observer 

 could have a few seconds rest between each zone, while the motion of 

 the mirror was reversed. If an object of interest was suspected, 

 it could be located by merely noting the time at which it was seen. 

 The right ascension would be given directly, and the declination would 

 be found by interpolation from the time of beginning and ending the 

 sweep. An examination of the object and a determination of its exact 

 location should be made on another evening. 



II. Measures of position. For many purposes positions could be 

 determined with this instrument as in a transit circle. It would gen- 

 erally be better, however, to make the measures differential, leaving the 

 mirror at rest and observing the transits of the object to be determined 

 and of two or more companion stars. The method of the ring mi- 

 crometer might be employed, or some modification of that with inclined 

 lines. In the latter case the zero of position could be found by the 

 transit of preceding stars, by setting the reticule by a divided position 

 circle, or perhaps better by keeping it in a fixed position, determining 

 the direction of the lines once for all, and applying a correction for 

 the declination of the object observed. Stars could be compared dif- 

 fering nearly a degree in declination, as the eyepiece could be moved 

 without danger of disturbing the reticule. For the same reason the 

 star could be followed for three or four minutes, and its transit over a 

 great number of wires observed. It is here assumed that the distortion 

 produced by the mirror is not very great. A slight distortion would do 

 little harm, as it would be the same for all stars of equal brightness. 

 If the stars differ greatly in brightness, the observer should determine 

 his personal equation between them in any case, and the same opera- 

 tion would eliminate the effect of the distortion. The large aperture 

 of the instrument would permit the observation of stars quite beyond 

 the reach of any meridian circle. The faintest asteroids could thus be 

 readily measured, and could probably be followed in many cases on 

 successive evenings to their stationary points. Zones of stars could be 

 observed very conveniently for the formation of charts or catalogues, 

 for the discovery of asteroids, stars with large proper motion, etc. 



