Astronomical Society. 379 



or coma :— a simple nebulosity. M. Pons's observations on 

 the comet, and some stars near it, from the 25th to the 28th of 

 Juh^, accompany this communication ; but as the comet was, 

 even then, rapidly approaching the Sun, they need not now 

 be recorded. 



Mr. Herschel submitted to the inspection of the members 

 present, a new double image micrometer, by Professor Amici 

 of Modena. The duplication of the image in this ingenious 

 instrument is effected by interposing a divided concave lens of 

 very long focus between the object and eye-glasses of a refract- 

 ing or between the mirror and eye-glass of a reflecting teles- 

 cope. The separation of the centres of the two segments is 

 performed by a rack and pinion sliding them on one another, 

 their edges being preserved in contact, and is measured on a 

 divided scale with a vernier in the usual manner. It is ob- 

 vious that each segment will form a separate image of a distant 

 object, more remote from the object end of the telescope than 

 its principal focus, and the value of the parts of the scale is 

 easily had from the expression, 



v x _£?L or 0-000004-8481 X -^- 



where a is the focal length of the object lens or mirror, <p' of 

 the bisected lens, and « the distance of the latter from the 

 former, all expressed in parts of the same scale into which the 

 micrometer is divided. This expression gives the number of 

 parts of such scale which represent 1" of angular measure, and 

 conversely, the value of one part of the scale, in seconds, will 

 be the reciprocal of this fraction. 



This micrometer possesses several advantages, among which 

 may be reckoned, simplicity of construction and use ; the mag- 

 nitude of its scale, which may be increased to almost any ex- 

 tent by an increase of the focal length of the bisected lens W ), 

 and lastly the advantage of dispensing with a table of reduc- 

 tion, by so fixing the place of the divided lens as to render one 

 part of the scale correspondent to 1" of angular measure. In 

 fact, if we equate the above expression to unity, we get 



« = p [1— 0-0000048481 Xip'} 

 and if this be the distance of the divided lens from the object 

 glass, one part of the scale will correspond to one second. Mr. 

 H. in his visit to Modena, in the spring of the present year, had 

 an opportunity of trying this instrument as applied to celestial 

 objects, in one of M." Amici's superb reflecting telescopes oil 2 

 inches ape. ture and 8 feet focus. The following, among other 



iik asures, were taken. 



3 J} 2 Distance 



