ASTEONOMY. 381 



it was photograplKMl by Taccbini and Schuster, aud a diawiiiy, made 

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

 incident tliiit induced Herr Holetschek to determine the conditions 

 that must be fulfilled by the orbit of a (;oinet that remains concealed 

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

 mined simply bv the formula J= -,— ^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 ijerihelion cannot be greater than from O-OG to 0-12 for an elon- 

 gation approaching 15°, certainly not for one of 22-5° (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=().00, 

 /=:106°). The Sohag comet belongs very jn-obably in the same cate- 

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

 (Radau, Bull, ^.s-fron., July, 1885.) 



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

 nent director of the Breslau Observatory, has communicated to the 

 Nachrichten* (Nos. 2GG5 and 2GG6)t a most valuable summary of the 

 orbit elements of comets from 18G0 to 18G4, with a similar list of newly 

 comi)uted 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. 



Dr. Galle's paper has been reprinted in Sirius, vol. 18, and a transla- 

 tion of the ])ortion relating to comets since 1860 has appeared in the 

 i^idereal Messenger (vols. 4 and 5). The present list is supplementary to 

 the catalogue given in the third edition of "Gibers' Methode 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 ul)er die Babn-Elemente der eeit dem Jahre 1860 exschieneueu Conieten, 

 sowie iiber ueu berechnete oder verbesserto Bahnen von Cometen der friihcren Zeit. 

 tSee also Aatron. Nachr., 2692. 



