40 



THE AMERICAN MONTHLY [February, 1880.] 



(2.) This question is answered by Prof. 

 Abbe in a paper read before the Royal 

 Microscopical Society, June nth, 1879, 

 and published in the December number 

 of their journal. To this article J. W. 

 and others interested are referred for a 

 more thorough discussion of the subject 

 than is possible in a brief space ; sufifice it 

 to say here that the defect spoken of is 

 due to a certain amount of residual spher- 

 ical aberration (Prof. Abbe calls it " chro- 

 matic difference of spherical aberration," 

 /. e., a. difference of the spherical aberra- 

 tion of the red and blue rays of the spec- 

 trum), which cannot be corrected by the 

 forms of construction now in use. Hence 

 it happens that an objective which is well 

 corrected for the marginal rays will be 

 under-corrected for the central ; or if 

 colorless by central light, will be over- 

 corrected by oblique. E. P. 



Philadelphia, January 19th, 1880. 



(2.) J.W. asks " why objectives made for 

 the resolution of test-objects are more 

 under-corrected than those intended for 

 ordinary work ? " Are they all so made ? 

 Has J. W. seen all the objectives made 

 for resolution ? Makers do not usually 

 give their reasons for making their object- 

 ives in special ways. Will J. W. define 

 " ordinary work ? " His ordinary work 

 may not be mine, or yours. 



Carl Reddott. 



MICROSCOPICAL SOCIETIES 



SAN FRANCISCO. 



The regular meeting of the San Fran- 

 cisco Microscopical Society was held 

 Thursday evening, December 5th. 



Mr. H. G. Hanks referring to a mineral 

 specimen exhibited by him a year ago, 

 stated that it is determined by A. Michel 

 Levy, M. E., of Paris, to be composed of 

 glaucophane, emaragdite and garnet. 

 The specimen was found near the " Wall 

 St. Mine," Lake County, Cal., in serpen- 

 tine, and is abundant in California, though 

 heretofore it has been known to exist no- 

 where but at the Island of Syra, Grecian 

 Archipelago, where it occurs in mica 

 schist. A description may be found in 

 Rosenbush's Mineralogy. Mr. Hanks 

 also exhibited a section of andalusite, 

 stating that the impurities forming the 

 regular figures characteristic of that mine- 

 ral are discovered to be magnetite. 



Mr. Wm. Ashburner read, for M. Chas. 

 Stodder, of Boston, a paper entited : 



" Note on Diatomaceas from Santa Mo- 

 nica, Cal.," which is too long for publica- 

 tion here, and it cannot be condensed to 

 advantage. 



O 



Exchanges. 



[Exchanges are Inserted in this column without 

 charge.] 



Polyzoa, Palates of MoUusca, Foraminifera, Sponge 

 and Holothurian-Spicules, Synapta-Anchors, Chirod- 

 ota- Wheels, Echinus-Spines, Minute Sea Shells, in ex- 

 change for Polyzoa, Foraminifera, Histological and 

 Pathological preparations, eic. Also, a quantity of 

 Sea Shells in exchange for Land and Fresh- Water 

 Shells. 



F. M. HAMLIN, M. D., 



Auburn, N. Y. 



J. J. B. Hatfield. Arsenal Avenue, Indianapolis, 

 Indiana, will exchange Bleached Plant-Sections for 

 good Diatomaceous material. List furnished on ap- 

 plication. 



For fine specimens of Tingis hyalina, send a 

 stamped envelope to 



ALLEN Y. MOORE, 



186 Dodge St., Cleveland, Ohio. 



Pleurosigmas and other unmounted Diatoms, both 

 Fresh Water and Marine ; also Marine Algae, Ferns, 

 and a large quantity of miscellaneous microscopical 

 material in exchange for good mounted objects. 

 M. A. BOOTH, * 



Longmeadow, Mass. 



To exchange, well-mounted Slides of Hair for 

 mounted or unmounted specimens of hair from the 

 rarer animals ; lists exchanged. 



WM. HOSKINS, 

 208 S. Halsted street, Chicago, 111. 



Nicely mounted Slides (opaque) of the Coal-tar 

 Derivatives, chemically prepared, crystalline in form, 

 showing their beautiful colors, in exchange for other 

 well-mounted material. Chemical name and formula 

 attached to each Slide. About a dozen varieties on 

 hand. 



F. L. BARDEEN, M.D., 

 3oJ^ Meigs street, Rochester, N.Y. 



Vanadate of Ammonia, (N H *) ^ V O *, Slides for 

 the Polariscope in exchange for other Slides. 

 H. POOLE, 

 Practical School, Buffalo, N.Y. 



Slides mounted from injected Frog's Gall-bladder, 

 Liver, Kidney, Stomach, Intestine and other parts, 

 exchanged for other good and interesting prepa- 

 rations. 



C. BLASDALE, M. D., 



Jericho, Queens Co., N. Y. 



Erratum. 



On page 5, line 10 from foot, for " ro- 

 tifer" read " species." Ophrydiwn bears 

 no close relationship to the Rotatoria. 



