121 
the unaided eye; while with proper appliances, it resolves the 
sodium line, D, and reveals that of nickel between the two. 
This is regarded as an excellent result with some of the best 
four-prism instruments. The lines are made visible without 
a sht, which at first sight seems impossible, by the fact that 
the beam of light is refracted at such an angle that it is re- 
duced to.an extremely narrow band, like that which passes 
through an ordinary sit. The value of this instrument for 
many uses, especially in the lecture-room, will doubtless be- 
come very great. 
Prof, Eaton illustrated his paper by numerous drawings 
on the blackboard, and also exhibited several of his instru- 
‘ments, with slight modifications of construction and results. 
Pror. C. A. SEELEY presented illustrations of a “ New 
System of Filtration.” The ordinary mode of filtration is 
defective, in that one-half of the paper employed is not util- 
ized, and that the process is somewhat obstructed by contact 
of the paper with the funnel. These and other objections 
are obviated by the devices proposed. For the funnel, a 
tube is ased, an inch or more in diameter, and about ten 
inches long. The lower end of the tube is covered with fil- 
tering paper, held in place by a piece of Swiss muslin 
fastened by a rubber ring or stout twine. The upper end of 
the filter-tube has a rubber cork, carrying a small rubber tube 
’ which leads to a simple pressure apparatus. To support the 
tube, a rubber ring, fitting it, is stretched over an upright 
rod or peg set into the table or shelf. The rapidity of filtra- 
tion may be changed at will by varying the diameter of the 
tube and the pressure. 
Pror. AuBerRT R. LexEps described a method which he 
had recently devised for the rapid and complete “ Purification 
of Mercury.” It consists in the filtration of the mercury un- 
der atmospheric pressure, through acidulated water and plugs 
of cotton wool, so as to remove entirely all the dust, dirt, and 
foreign metals, as well as the water or other liquids, and to 
render large quantities of mercury perfectly dry and clean in 
a very short time. 
iv. 
