156 



THE AMEKICAN MONTHLY 



[August, 



Elmira, is that pertaining to the size 

 and nomenclature of eye-pieces. 

 Looked at from a purely business 

 point of view, we are sure that, how- 

 ever desirable it may be to secure 

 uniformity in size and nomenclature, 

 and however strongly the advantages 

 may be presented by the Committee 

 having that matter in charge, no ac- 

 tion the Society can take will alone 

 bring about the desired uniformity. 

 Manufacturers of microscopes, with 

 perhaps two or three exceptions, will 

 not change their machinery at the dic- 

 tates of any Society. The only way 

 to secure uniformity is to convince 

 purchasers of its importance. Never- 

 theless, this is no argument to dis- 

 courage the Society from adopting a 

 standard of uniformity, for the result 

 can only be brought about by some 

 initiatory action from some influen- 

 tial body. We look for the report of 

 the Committee with great interest. 

 o 



Table of Numerical Aperture. 

 — Last month we printed a table of 

 numerical aperture, and a microme- 

 tric table, but as it is not always con- 

 venient to refer to the Journal to 

 examine the table, we reprint them 

 this month on one of the advertising 

 pages, so that they may be torn out 

 for use. 



It may be that some readers do not 

 fully understand the aperture table. 

 We do not consider it advisable to 

 explain it in detail, for the subject of 

 numerical aperture has already been 

 discussed quite freely in these col- 

 ums; but if there is anything still ob- 

 scure about the meaning of the table, 

 or concerning the relation between 

 angular and numerical aperture, we 

 will endeavor to make it clear to any- 

 one who will plainly state the diffi- 

 culty. 



It will be convenient to remember 

 a very simple relation between the 

 metric and the English raicrome- 

 tric measures, which is expressed as 

 follows : — 



.001 cm. = .00039370 inch. 

 gg^flO inch = .00040000 inch. 



The difference between these is, 

 .00000630, an amount quite unworthy 

 of consideration for ordinary micro- 

 metric purposes, so we may say that 



.001 cm. ^(HF = or .0005 of an inch. 

 o 



Preservation of Protozoa. — A 

 thoroughly good method of preserv- 

 ing minute organisms would be of 

 great value to microscopists. The 

 process of A. Certes, by means of os- 

 mic acid, is effectual in many cases, 

 but not always satisfactory. 



Eugen Korschelt,* describes the 

 following method, which he regards 

 as better than that of Certes: A very 

 small drop of water containing the in- 

 fusoria is placed on the slide and 

 covered with the thin glass. A i per 

 cent, solution of osmic acid is then 

 allowed to flow under the cover, and 

 drawn off at the opposite side. It is 

 followed by water, and then by 70 

 per cent, and 90 per cent, alcohol, 

 and finally by water. For coloring 

 the now fixed and hardened animals, 

 C. Weigert's picro-carmine is used. 

 It is allowed to act i ^-2 hours, when 

 it is removed and again 70 per cent, 

 and 90 per cent, alcohol are used, 

 followed by absolute alcohol, oil of 

 cloves and Canada balsam. 



This process, the author states, can 

 be carried out in a short time, and 

 gives fine results with many infusoria. 

 But for some, the amoebae for ex- 

 ample, the osmic acid will not do. 

 For amoebae therefore, a 2 per cent, 

 solution of chromic acid is used in- 

 stead, the other manipulations being 

 the same as for osmic acid. The 

 chromic acid should act 2-3 minutes. 



In this way many infusoria and 

 flagellates have been preserved with- 

 out the slightest shrivelling ; the 

 cilia and vacuoles remain as in life, 

 and the granules and nuclei are col- 

 ored intensely red. By far the most 

 surprising result is obtained with the 

 amoeboe; their form is fixed and the 

 vacuoles are to be seen even in the 



* Zoologischer Anzeiger, 1882, No. 109. 



