228 Astronomical Society. 



ber of times which the distance of the objects from each other is 

 contained in the distance of the observer from the nearer of them, 

 tuhen the angle at the nearer object is a right angle, the only case of 

 the problem which, in this application of his instrument, the inven- 

 tor appears to have had in contemplation. 



A method of making the necessary computations for deducing 

 the longitude from an occultation of the moon, by Lieut. C.ll. Drink- 

 water, R. N. was read. In the method proposed by Mr. Drinkwater, 

 the sun's right ascension, and the moon's declination, horizontal 

 parallax and scmidiameter, are taken from the Nautical Almanac 

 for the approximate Greenwich time; and the star's meridian di- 

 stance being found, the latitude and parallax are corrected for the 

 earth's ellipticity. 



Then, calling the reduced horizontal parallax P, the star's meri- 

 dian distance M, its polar distance p and the latitude / ; he com- 

 putes ai-c A, from tan A = cos M . cot I ; arc E, from B = ^ -i -^ ? 

 C, the principal effect of parallax in polar distance from C = P . 

 sin /. sin B . sect A, (— when A is less than p, otlierwise + , unless 

 M exceeds 6'', when it is always — } ; D the parallax in right ascen. 

 sion from D =P . cos / . sin M . cosect p ( — when the star is east of 

 the meridian, otherwise + ) ; and E, the final correction of declina- 

 tion from E = — " ' .^'" ^^ ^' always — . And having thus found the 

 sin I" 



declination of the point of occultation, the difference between it 

 and that of the moon's centre is known ; and from this difference, 

 the moon's scmidiameter, and the declinations of both objects. 

 Mr. D. computes in the usual way the difference between the right 

 ascension of the planet; and hence finds the right ascension of the 

 moon's centre : from which, by interpolating in the Nautical Al- 

 manac, he finds the Greenwich time. 



The business of the evening being concluded, ProfessorAmici( one 

 of the Associates of theSociety), at the request of the President, obli- 

 ghngly permitted the inspection of several instruments of his inven- 

 tion and workmanship. The principal were, anew compound achro- 

 matic microscope, a reflecting circle on a new construction, and a 

 prismatic refl(3cting sextant. The microscope is formed with three 

 achromatic double object-glasses, applied separately, or in combi- 

 nation ; their common a.xis is directed vertically downwards, and the 

 j'efracted pencil is turned into a horizontal position by internal re- 

 tlexion at the base of a right-angled prism, for the convenience of 

 observation, and after passing along the axis of a tube about seven 

 inches long, received on the eye-piece. The object is powerfully 

 illuminated from below by the reflected light of an Argand lamp, 

 or dayligiit, collected in tiie focus of a large concave mirror. 

 Several objects were viewed, such as tlie featliers on the wing of a 

 butterfly; the pollen of the mallow, &c., which were shown with 

 the utmost distinctness and beauty with powers of 700 and 1500* 



(the 



* These powers, however extraordinary they may seem, are not exay;- 

 geratcd. The writer of this notice has since, more than once, witnessed, 



and 



