ASTRONOMY. 381 



it was photographed by Tacchini and Schuster, and a drawing made 

 of its tail by Trdpied; but the comet was not seen again. It was this 

 incident that induced Herr Holetschek to determine the conditions 

 that must be fulfilled by the orbit of a comet that remains concealed 

 by the sun's rays during the time that its absolute brightness (deter- 

 mined simidy by the formula J=--— -) is sufficient to admit of its dis- 



covery. 



Herr Holetschek judges from the examples furnished by the comets 

 of the last ten years that the magnitude of a comet remaining invisible 

 at its perihelion cannot be greater than from 0-06 to 0-12 for an elon- 

 gation approaching 15°, certainly not for one of 22'5o (the calculation 

 was made for these two elongations). In order that it may remain vis- 

 ible in the two branches of its orbit, its orbit must offer a certain sym- 

 metry in reference to the earth's radius vector ; the heliocentric latitude 

 of the perihelion must then be sufficiently small, and the comet must be 

 in conjunction with the sun at the time of its passing its perihelion. 



These results, which are not at all rigorous, suffice to show that the 

 unobserved perihelion transits of comets may not be as infrequent as 

 would have been supposed. We know of periodic comets whose re- 

 turns sometimes elude observation. But there are others which would 

 have always remained unknown if their transit had occurred at any 

 other time of the year; such, for example, is the comet of 1821 (g=0.09, 

 /=106O). The Sohag comet belongs very i)robably in the same cate- 

 gory as this last; its perihelion distance must have been very small. 

 (Radau, Bull. Astron., July, 1885.) 



Guile's svpplementory list of recent comets. — Dr. J. G. Galle, the emi- 

 nent director of the Breslau Observatory, has communicated to the 

 Nachrichten* (Nos. 2065 and 2666)t a most valuable summary of the 

 orbit elements of comets from 1860 to 1864, with a similar list of newly 

 computed orbits of comets earlier than 1860. Brief notes generally re- 

 lating to the discovery and period of visibility of the comet, and to the 

 elements given, accompany each orbit. 



l)r. Galle's ps^per has been reprinted in Sirius, vol. 18, and a transla- 

 tion of the portion relating .to comets since 1860 has appeared in the 

 Sidereal Messenger (vols. 4 and 5). The present list is supplementary to 

 the catalogue given in the third edition of " Olbers' INIethode zur Be- 

 rechnung der Cometenbahnen," so that taken in connection with the 

 latter it forms the most complete catalogue of comets that is now 

 available. At the end of the year 1884 the number of different comets 

 of which orbits have been computed was 302. The number of appari- 



* Ueberischt iiber die Bahn-Elemente der seit dem Jahre 1860 exscbieueneu Cometen, 

 sowie liber neu berechnete oder verbesserte Babnen von Cometen der friiheren Zeit. 

 tSee also Astron. Naclir., 2692. 



