1867.] Astronomy. 233 



or two, give the business that element of discipline and unity of 

 action which volunteer observers, collected from various parts of a 

 large country, will almost necessarily want." 



Mr. Peacock discusses the epoch of a partial eclipse recorded on 

 a brass plate dug up at Calcutta, and described as having occurred 

 " in the month Chaitra, when the sun was entering the northern 

 hemisphere, the moon being in the Nakshatra Aswini." He identi- 

 fies it with the eclipse observed at Constantinople on the 3rd of 

 April, a.d. 889. 



Mr. Wray presents an interesting paper on the correction of 

 the secondary spectrum of object-glasses. After trying " a great 

 number of different kinds of glass," flint and crown, having 

 densities varying from 2 # 833 to 5*49, he came to the conclusion 

 that by the mere combination of glasses, the secondary spectrum 

 could not be corrected, " with anything like a reasonably shallow 

 system of curves, even where four lenses were used." He finds, 

 however that by a judicious selection of flint and crown glass, 

 separated by an extremely thin meniscus film of highly dispersive 

 cement, it is easy, not only to destroy the irrationality, but even to 

 invert the spectrum. Under high powers he obtains a perfectly 

 achromatic image of the moon and planets, " which are shown in a 

 surprisingly sharp and clean manner on the black ground of the 

 sky, reminding one of a first-class reflector, under its very best 

 behaviour. The Astronomer-Eoyal has promised to examine a large 

 object-glass for Mr. Wray, who has constructed one for the pur- 

 pose, having a 7-inch clear aperture and a focal length of 8 \ feet. 



Mr. Knott exhibits in a table the apparent increase of diameter 

 shown by star-discs as the aperture of the telescope is diminished. 



Mr. Dawes remarks, with reference to the eye-piece prism 

 suggested by him for correcting errors in double-star measurement, 

 that in all ordinary cases he should not recommend its use ; " only 

 in those where the obliquity is such that a slight and not incon- 

 venient inclination of the head was insufficient to bring the stars 

 into an apparently vertical position." 



The small companion of £ Herculis having emerged from its 

 recent conjunction with its primary, Mr. Dawes has succeeded in 

 effecting measurement of it. One set of measures was taken exactly 

 at noon on December 30th, the sky being quite unclouded. Mr. 

 0. Struve, with the great Pulkowa refractor, found the object one 

 of extreme difficulty, and was surprised that Mr. Dawes should 

 have been enabled to see, much more that he should have measured 

 the object with the smaller telescopic powers at his disposal. The 

 measures of Mr. Dawes agree admirably with M. Struve's. 



