182 OBSERVATIONS ON NEBULiE 



believe, of platina; with the present telescope, they are each 2". 28 in thickness: 

 their number and arrangement, as also the construction of other parts of the 

 micrometer, is very similar to that of Troughton's spider-line micrometer, as 

 described in Pearson's Practical Astronomy. 



21. The illumination, however, is different, and resembles that employed by 

 Frauenhofer to render visible his lines on glass. The light falls on the wires 

 at right angles to the optical axis of the telescope, thus illuminating them upon 

 one side, while the rest of the field is dark. This arrangement possesses con- 

 siderable advantages in rendering visible small stars, which it would be other- 

 wise extremely difficult to measure, if, indeed, they could be seen at all in an 

 illuminated field. I found this property of great use, since nearly all the stars 

 I wished to measure were below the tenth or eleventh magnitude. The chief 

 faults of the present construction were two. The scale of teeth in the field of 

 view, which marks the number of whole revolutions, is illuminated only edge- 

 wise by the light; it was in all positions difficult, and in many impossible to 

 be read off. Great loss of time, and sometimes impaired accuracy, was the 

 necessary consequence. The other defect was in the size of the wires; this 

 made the line of illumination needlessly broad, and prevented small stars from 

 being seen near or on it, unless too faintly lighted for accuracy. Neither of 

 these faults are necessary to this construction, and both might be remedied 

 without difficulty. 



22. The telescope, being mounted with an altitude and azimuth motion, 

 which, moreover, was quite unsteady and liable to tremors, was ill-adapted to 

 micrometrical observation. The mode of managing the micrometer was ne- 

 cessarily peculiar, and its essential differences from the common and more 

 regular employment of the instrument I shall briefly describe. The circle of 

 the micrometer was turned until the stars in the field ran parallel to the two 

 moveable wires. Their transits across the fixed wire, which then represented 

 an hour circle at that point of the heavens, were noted blindly in a book at 

 hand, no other way being practicable with a succession of stars at intervals of 

 a few seconds each, or occasionally of less than a second. These furnished 

 differences of right ascension. Those of declination were obtained by bringing 

 the moveable wires over the two stars whose difference was required. No 

 measure was considered good in which some star, brought on one of the wires 

 for that purpose, was not completely bisected during the whole run across the 



