ASTRONOMY. 379 



lied similar iuvestigations mucb further. At first, however, they were 

 unable to obtain clear images of stars of the 2d magnitude, while 

 now it is possible to print those of the 14th, or, in other words, to 

 transfer to paper an image produced by an object only a hundred-thou- 

 sandth part as bright as formerly. Professor Pickering's researches 

 have been carried on with an instrument that he has devised himself, 

 in which a Voigtlander portrait lens of 8 inches aperture and 44 inches 

 focus has been reground and mounted equatorially. It is driven by 

 clockwork, having a Bond spring-governor, controlled electrically by a 

 sidereal clock. 



We have already referred (under Nebulae) to the fact that Professor 

 Pickering has found upon one of his plates a trace of the nebulae around 

 Maia, discovered by the Henrys at Paris. 



Stellar pliotography at the Cape Observatory. — Upwards of one hundred 

 successful photographs have been obtained towards the completion of a 

 photographic Durchmusterung of the southern heavens. Each photo- 

 graph covers a square of CO on the side, and shows all stars similar iu mag- 

 nitude to those of Argelander's Durchmusterung. The lens employed 

 is a "rapid rectilinear lens," composed of two combinations, having an 

 aperture of 6 inches and a focal length of about 52 inches. The exposure 

 of each plate is one hour, and the plates are the most sensitive "Paget 

 dry plates," specially made for that purpose. Mr. Finlay, of the Cape 

 Observatory, stated at a meeting of the Liverpool A stronomical Society, on 

 October 13, 1885, that a 9-inch lens had been ordered and would soon be 

 ready. It is proposed to divide the whole southern hemisphere into sixty 

 squares, each one overlapping another adjoining it. Two plates are taken 

 of each picture, so as to avoid any mistake. Th^ work will take from 

 three to four years, and will comprise 1,000 pictures. Mr. C. Ray Woods 

 has been put in charge of this work, and he intends to continue at the 

 Cape the work of photographing the corona, which he lately undertook, 

 under Dr. Huggins' direction, in Switzerland. 



COMETS. 



Theory of comets^ tails : Les syndynames et les synchrones dans les co^ndtes, 

 25 i)p., 1 plate, 4to. Les syndynames et les synchrones de la comete Pons- 

 Broolcs. 24 pp., 1 plate, 4to. — " M. Bredichin, while sharing some of Bes- 

 sel's ideas concerning the existence of a repulsive force, has succeeded 

 in developing a complete theory concerniug the tails of comets; a theory 

 which accounts very satisfactorily for most of the observed phenomena. 

 M. Bredichin applies the name syndyname to the parabolic curve in which 

 the particles of the tail that have successively left the nucleus under the 

 influence of a given repulsive force /i, are disposed at the moment of 

 observation. The synchrone is the curve in which occur the particles 

 that have left the nucleus at the same time under the influence of sev- 

 eral forces //, /<', .... 



