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tion or polar distance, to which the assistant will set the 

 sliding- weight at the same time that the telescope is set on an 

 object. It has been ascertained by trial that the elasticity of 

 the impelling band (100 feet long) is quite sufficient to equal- 

 ize the intermitting movement of such a scapement. 



In searching for known objects, there is, of course, occa- 

 sional difficulty in finding them, from the small field of view 

 of ordinary eye-glasses. This is remedied by a supplemental 

 eye-piece of very wide field ; a slide carries it, and the holder 

 of the others, so that by a little shift one can be substituted 

 for the other in an instant. The eye-piece is similar to one 

 which Dr. Robinson had long since made for his own instru- 

 ment. It consists of three lenses ; and fulfils the three condi^ 

 tions of equal flexure of the pencils, achromatism, and flat field, 

 while its distinctness is equal to a Huygenian of equal power. 

 In this one the field-glass is six inches diameter ; it magnifies 

 216 times with a field of thirty-one minutes ; and though this 

 will only bring into action forty-three inches of the mirror, 

 yet even so its optical power is very great ; and Dr. Robinson 

 thinks the view of the moon in it the most magnificent spec- 

 tacle he ever saw. A nebula is easily found in this wide field ; ' 

 and bringing it into the centre, the eye-slide is shifted, and it 

 is viewed with the higher powers. 



The micrometer also appears deserving of notice. Not- 

 withstanding the prodigious light of the telescope, it was found 

 that any illumination of wires extinguishes many of the fainter 

 details in nebulse. Lord Rosse, in drawing 51 Messier, used 

 a very ingenious substitute, a screw whose threads were rubbed 

 with phosphorus. Dr. Robinson had made experiments with 

 a micrometer whose platina wires were faintly ignited by a 

 voltaic current ; he found, however, that the heated air pro- 

 duced tremors quite incompatible with the use of high mag- 

 nifying powers. An experiment of Mr. Babinet, described in 

 the Comptes Rendus, has suggested a contrivance which 

 seems quite satisfactory. If light be admitted through the 



