xcviii GENERAL SUMMARY OF SCIENTIFIC AND 



MICROSCOPY. 



Objectives. Professor Ruell Keith, of Washington, has dis- 

 cussed the formula of an immersion objective made by Tolles, 

 of Boston, and proves that it has a greater angle than cor- 

 responds to the maximum possible for dry objectives. The 

 extreme aperture in fluid balsam, no allowance being made 

 for the setting of the small front lens, is 110 35'; or, allow- 

 ing 0.00162 for the setting of the front lens, the aperture is 

 reduced to 87. The objective was composed of seven lenses, 

 and had an equivalent focus of one tenth of an inch. 



In the American Naturalist for July, 1874, Mr. George 

 W. Morehouse gives a somewhat extended account of the 

 marvelous performance of a one fiftieth of an inch objective 

 made by Mr. Tolles. The most severe tests were easily re- 

 solved, and the lines of Amphipleura pellucida shown as rows 

 of dots. 



The Boston Journal of Chemistry announces that Mr. 

 Tolles has alone made an objective one seventy -fifth of an 

 inch; but that journal is evidently unaware that Messrs. 

 Powell & Lealand have long since achieved a similar result. 

 The power of this objective is such that a single white cor- 

 puscle of the blood covers the entire field of vision. 



Mr. Henry J. Slack speaks very favorably of the recent 

 objectives made by Zeiss, of Jena; especially of his low- 

 priced one-sixth inch. Zeiss makes three immersion glasses, 

 the real angles of w T hich, for water, are, as he states, between 

 104 and 108; whereas theory and practice agree in assign- 

 ing an angle of 105 as the limit that may not be exceeded 

 in dry systems without rendering the correction of spherical 

 aberration impossible, or reducing the working distance so 

 that the systems become exceedingly troublesome to use 

 an angle of 97 for water would imply an angle of 180 in 

 air. 



The objectives of L. Beneche, of Berlin, are very highly 

 6poken of by Mr. W. J. Hickie, the one-fourth inch resolving 

 P. angulatum with direct light, and without a condenser, in 

 the most satisfactory manner; and with oblique light easily 

 resolving Surirella gemma. 



Microscopical Apparatus. The Hoi man Siphon Slide, as ex- 

 hibited by Dr. Richardson at the Microscopical Section of 



