64 ANSWERS TO CORRESPONDENTS. BOOKS RECEIVED. 
Mr. Epwarp Wuymper has arrived at Copenhagen from his 
second exploration of W. Greenland. He brings with him rich 
collections of curiosities, and some singular specimens of fossil 
wood. 
Proressor AGassiz has recently been elected a foreign associate 
of the French Institute (Academy of Sciences). It may be re- 
membered that the number of foreign associates of the Academy 
of Sciences is limited to eight. 
We are glad to learn that Mr. Charles Stodder has saved from 
the conflagration of November 9th, all his valuable stock of Tolles’s 
telescopes, microscopes, and microscopic objectives. Work in the 
shop will go on, and all orders filled as usual. 
ANSWERS TO CORRESPONDENTS. 
- M., Penn.—The glass paraboloid, “ Wenham’s Parabola,” has seen rominently 
rought forward as a means of ia illumination with high powers, by Chevalier 
Husttens d de Cerbecq, of Brussels. As thus praf t gives excellent definition with a 
well co ted lens, but es com npletely with a poorly corrected one.— I. W. 
would probable be undesirable to have all Aoin es mounted 
brasswork of exactly the same length measured from the focal paw of the objective 
to the top of the mountin e convenience attained in Working with the straight form 
a double-nose piece sai not compensate for the ¢ disadvantage of wearing the rack 
tah entirely at oep point. A general Peep pndas of length of compound body, how- 
ever, is greatly to be desired.—R. H. W. 
HW By Diak —l. big best microscopical definition attainable at present is by 
means or‘ immersi ion” Jens 2. “they 4 are poor as durable as dry lenses, since 
both will las st, with “frequent b "but careful use, until rendered obsolete by the improve 
> The Peb re li a 
other objectives to injury by enkar d temper aue ete. 4. Several first-class makers 
have n eed Tor many year 1 immersion eng a can be 
$ eseanity irk nsferred, y 8 screw-collar ‘adjustment, to Hina oo objectives. Thus t e owner 
= haa a choles of erie, wi g disadvantage.— R. H. W. 
sanstalt. Nos. 1-6, 
en No.1. yia Aniu. 1872. 
sae ee 1370-71, Pe °is72. 
i e 68. si al iste, ‘4to. 
fzun; i weg se der’ Wissenschaften, Math. Naturw, Classe. Bd.lxiv. Abth. 
eft 1-v. ro a ‘Hert i- Wien. 
schriften der K. Ak sbidonde ar Wissenschaften, Math. Naturw. Classe. Ba, 31. Wien, 
e py aa 
E siinnd na sherichte der Naturwissensch. Gesellschaft Isis in Dresden, 7 1872.. Jan.- Mareb. 
1, 1872, 
~ Memoires de la Societe Royale des enra ay du Rigs g 1866-71, 
L PPRA E and Historical rebec, Prats tee aioe “ayo, 
of the the Boston aa pamit History, ol, 14, pages si ue 8vo. 1872, 
: pea ara p rira oN ‘atural History. Vol. 2, 2, part 2, No.3; part 2, No.1; and 
3 of the gree of Natural PNS of Philadelphia. Part 2. May and Sept., 
the Origin of Species, By B. G. Ferris. 12mo, pp. 69. New Haven, Chat- 
i “Red He matite Ore of Bedford Co., Pen sh Kimball, 8v 5 -T 
Pee Te of i, Cisilairiachtes. dation. By J.P. Pki imballi. 8vo, pp. age de wa 14572. 
‘lodge ogee eg ae iy dt P. Kimball, ‘sv. 
aeni ee T Petites, Housel pia ete giguer. Paris, p Faris. No. 
