M. Steinheil's New Micrometer. 173 



on the existence of a membrane forming the coat of his boyau 

 or mass of cylindrical form ejected from the grain of pollen. 



I reserve, however, my observations on these and several 



other topics connected with the subject of the present inquiry 



for the more detailed account, which it is my intention to give. 



July 30, 1828. 



[The examination of the unimpregnated vegetable Ovulum, mentioned at 



the beginning of this Paper, will be found in Phil. Mag. vol. lxvii. p. 352.] 



XXVIII. On a New Micrometer, principally intended for the 

 Construction of a more complete Map of the Heavens, By 

 M. Steinheil. 



[With an Engraving.] 



To the Editors of the Philosophical Magazine and Annals. 

 Gentlemen, 



I" HAVE lately received, through the kindness of Professor 

 ■*■ Schumacher, one of Fraunhofer's 42-inch refracting tele- 

 scopes, with an object-glass of three inches diameter. This 

 instrument is a most excellent one, and far exceeds any tele- 

 scope of the same size that I have ever seen. Attached to it, 

 there is one of M. Steinheil's new micrometers; the first, I be- 

 lieve, that has ever reached this country. As a description of 

 this invention may be interesting to many of your readers, 

 I beg leave to send you the following translation (from No. 117 

 of Professor Schumacher's Astronomische Nachrichte?i) 9 with 

 which Dr. Tiarks has favoured me. 



I am, Gentlemen, your obedient servant, 



July 15, 1828. Francis Baily. 



The comparison of a celestial map with the heavens, for 

 the purpose of inserting therein the stars of inferior mag- 

 nitude, is subject to peculiar difficulties, as is well known to 

 every one who has undertaken this task. The method de- 

 scribed by Professor Bessel in No. 93 of the Astr. Nachr. is 

 indeed exceedingly simple, and on that account much to be 

 commended; but it is in one respect not quite satisfactory. 

 For the differences of right ascension and declination with re- 

 spect to the standard stars which determine the position of 

 the stars to be inserted, can only be estimated by means of the 

 cross wires; which is the more uncertain and unsatisfactory 

 the greater those differences become. A method therefore by 

 which these coordinates could be measured, appears to me to 

 be a very desirable object : and would have the additional ad- 

 vantage of doing away with the necessity of laying down the 

 stars by candle-light, an operation at once tedious and hurtful 

 to the eyes, as the observations may in the present case be re- 

 duced 



