457 



both the lens and the naked aperture. The magnified object may 

 thus be compared with a scale of any large dimensions at such a 

 distance as may best suit the convenience of the observer. 



The author, however, recommends a small scale attached to the 

 instrument, as better adapted for steady comparison with the object 

 to be measured. 



The instrument has externally the appearance of a telescope, con- 

 sisting of three tubes, with the little lens at its smaller extremity ; 

 and in the place of the object-glass is fixed the scale of equal parts, 

 which consists of pieces of wire placed side by side, and so propor- 

 tioned in their lengths at regular intervals, as to be easily counted. 



A wire of known dimensions, as for instance, -j-J-o-th of an inch, 

 being then placed in a suitable position before the lens, the tube is 

 drawn out till this wire apparently occupies fifty divisions upon the 

 scale, and consequently each division at that distance corresponds 



to of an inch in the focus. Again, at half that distance the 



50 X ^-00 



same wire covers only twenty-five divisions, each of which now cor- 

 responds with T-innrtn of an inch seen in the focus of the eye-glass. 



These numbers are marked accordingly on the outside of the tube, 

 and the intermediate fractions TroV-o-. , O 'o o , &c. are found by dividing 

 the exterior scale into equal intervals. Hence in the measurement 

 of any wire, the number of divisions which it occupies on the interior 

 scale are to be noted as numerator, and the number marked on the 

 tube externally as denominator of a fraction, expressing its dimen- 

 sions in proportional parts of an inch. Since the correctness of the 

 instrument depends on the precision with which the first wire is 

 known as basis of the exterior indications, the wire is made of fine 

 gold, and its dimensions determined by the weight of a given length. 



Observation of the Winter Solstice 0/1812, with the Mural Circle at 

 Greenwich. By John Pond, Esq. Astronomer Royal, F.R.S. Read 

 February 25, 1813. [Phil. Trans. 1813, p. 123.] 



The weather was so extremely unfavourable, that it was not pos- 

 sible to obtain more than eight observations of the sun, from which 

 the obliquity of the ecliptic at the late solstice could be deduced ; 

 from these it is inferred to have been 23 27' 47"'35, that from the 

 summer solstice having been 23 27' 51"'3. This small discordance, 

 it is observed, might be easily made to disappear by a slight modifi- 

 cation of Bradley's refractions ; but the Astronomer Royal has not 

 yet had an opportunity of making a sufficient number of observations 

 on circumpolar stars with the new circle, to warrant making any 

 corrections in his table of refractions, and he leaves the subject of 

 the discordance of the solstices for discussion in a separate paper. 



