17 



The interval between a principal and next following 

 minimum derived from the two epochs is 3o-709 days; the 

 value derived from the results given in my former paper is 

 84-886 days. The mean is therefore 'S5-297 days, which is 

 practically identical with the half of the mean period derived 

 from all the observations made since the discovery of the 

 star's variability — the difference being only O'Olo of a day. 



The mean magnitude of the star in the principal minima 

 in the years 1859 to 18G8 was 9'7o, and in the years 1878 

 to 1884, 9*14, or 0-61 higher; while on the other hand the 

 mean magnitude in the secondary minima in the former 

 years was 9"03, and in the latter 9-61, or 058 lower. 



No theory has yet been advanced that will account satis- 

 factorily for the ordinary phenomena of variable stars, and 

 it seems very probable that this occurrence of a reversion 

 of the minima of a double-period variable will increase the 

 difficulty of framing such a theory. 



MICEOSCOPICAL AND NATUEAL HISTOEY SECTION. 



November 10th, 1884. 



Thomas Alcock, M.D., President of the Section, 

 in the Chair. 



Mr. Mark Stirrup, F.G.S., exhibited specimens of the 

 nests of the Trap-door-nest Spider Nemesia coementaria 

 (Latr.) from Cannes and explained some of the interesting 

 habits of these creatures. 



They form subterranean cylindrical burrows, generally 

 penetrating from two to three inches from the surface of 

 the soil; the openings into these burrows or nests are closed 

 with a beautifully close fitting door, which opens on a hinge, 

 and when these doors are closed, it is extremely difficult to 



