CORRESPONDENCE. 241 



of his note a statement of offers or wants, and other members 

 addressing him directly by mail, in regard to the same. 



Instructions for Cleaning' Foraminifera of the Chalk. — 

 Mr. C. J. Muller kindly sends us the following note : — Having obtained 

 a quantity of the shells by the usual process of elutriation, mix it with 

 four or five times its bulk of silver sand which has previously been 

 well washed, and put the mixture in a long 2 or 3 ounce phial with a 

 sufficiency of water. Shake up the whole (not violently) for ten or 

 fifteen minutes. Let it rest for three or four minutes, and then pour off 

 the turbid water. Renew this operation as many times as you like. The 

 Foraminifera will always settle down last, and form a distinct stratum 

 upon the surface of the deposited sand. The sand when shaken up 

 with the shells will act as a gentle rasp, and remove from their 

 surface most of the hard granular particles which injure their appear- 

 ance. When the cleansing operation is completed, the water will 

 rapidly clear upon the mixture being set aside for three or four 

 minutes. 



There is no difficulty in separating the shells from the sand. Let 

 the whole quietly settle down ; pour off the clear water, and allow 

 the whole to rest for a few minutes. Now add a fresh supply of 

 water rather forcibly, when the shells will immediately rise, leaving 

 the sand below. The water with the shells must now be poured off, 

 before they have time to settle down, into another vessel where they 

 may subside. The deposit may then be dried and mounted in 

 dammar or Canada balsam in the usual way. 



COKRESPONDENCE. 



Professor Keith's Criticism. 



To the Editor of the ' Monthly Microscopical Journal: 



Sir, — I am quite willing to meet fair discussion relating to the 

 principle on which I have asserted that the measured apertures of 

 all object-glasses have hitherto been greatly in excess of the true 

 angle. 



Professor Keith's letter in your last issue appears somewhat 

 superfluous, as it does not in any way help to elucidate the point. 

 First, merely on his own judgment, he attributes " errors " to me — 

 not considering that such have yet to be proved. 



The remainder of the letter resolves itself into the assertion that 

 it is " possible to compute the spherical aberration of any com- 

 bination of lenses, and with any degree of accuracy." Very well ; I 

 shall be glad if this can be done, so that the tedious trial and error 



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