40 CORRESPONDENCE. 



It has coarse longitudinal ridges and furrows, and, it seemf?, slightly 

 radiating transverse ridges passing over and across the longitudinal 

 ones and through the furrows. 



Erratum in Professor Abbe's Paper. — Owing to a printer's error, 

 the word sum in the 19th line of p. 193 in the October number has 

 been placed instead of sine, which it should have been. The word 

 sum in the preceding line is a mere surplusage. We may state also 

 that Dr. Fripp had nothing to do with either the reduction or repro- 

 duction of the article in these pages. 



COKRESPONDENCE. 



Eesolution of Amphipleura pelluoida. 



To the Editor of the ^Monthly Microscopical Journal.^ 



Wayland, N.Y., October 23, 1875. 

 Sir, — That AmpMpleura pellucida is " dotted " or " beaded " can 

 no longer be doubted. It was so seen by Mr. J. Edwards Smith, 

 of Ashtabula, Ohio, and the observations published in the ' Lens ' 

 (Chicago) for April, 1873. The objective used was a Tolles' yV^h, 

 I had at nearly the same time, and independently, reached the same 

 result with a Tolles' ^'oth immersion (see ' American Naturalist ' 

 for May, 1873). In the ' Boston Journal of Chemistry ' for June, 

 1875, Samuel Wells, Esq., of Boston, confirms these observations, 

 using a Powell and Lealand ygth and a Tolles' ^th and y^jth. And 

 now Messrs. Dallinger and Drysdale, in the ' Monthly Microscopical 

 Journal' for September, 1875, announce the same result, obtained 

 with a Powell and Lealand ith. Witnesses enough. 



A word as to objectives of medium angle of aperture. Some 

 microscopists seem to have the impression, both in England (recent 

 discussions in this Journal) and in America (' Popular Science 

 Monthly,' New York, November, 1875), that such glasses, of good 

 quality, can only be obtained from Germany and France. But the 

 principal makers of England and America can make, and some of 

 them, at least, have been making this class of objectives when re- 

 quired, of proportionate merit with their higher-angle immersion 

 lenses. Kecognizing the fact that no one glass can bo adapted to 

 all grades of work, they undertake to meet the requirements of 

 those interested in any department of microscopic study. As a case 

 in point, I have a Tolles' ^th inch, dry, of 70° angle of aperture, 

 with cover adjustment, costing ^20. 00 IJ. S. currency. After allow- 

 ing for depreciation of paper currency, this is about 4Z. 12s. (In 

 comparing prices with those of foreign makers, duty, exchange, &c., 

 "would be considered.) This lens has a tapering front, permitting the 

 use of a bull's-eye condenser on opaque objects, and has sufficient 



