cxxxviii GENERAL SUMMARY OF SCIENTIFIC AND 



and elimination of errors, the objectives of Zeiss, e. g. } a one- 

 quarter inch of forty-eight degrees, and a one-sixth inch of 

 sixty-eight degrees, will perform work, as Mr. Slack proves, 

 hitherto supposed to be only within reach of the most ex- 

 pensive large-angle objectives. Zeiss has, so to speak, min- 

 imized angles of aperture, and secured great working dis- 

 tance and penetration, and yet obtained the amount of sep- 

 arating and resolving power of much larger angled object- 

 ives. Mr. Slack truly observes that opticians have been en- 

 couraged to make excessive apertures substitutes for good 

 corrections, and that naturalists and jmysiologists have been 

 too contented with feeble resolving powers, under the belief 

 that any more capacity for resolution must mean less pen- 

 etration. 



Not indirectly connected with this subject of large angle 

 is the " Measurement of the Moller Probe-Platte," by Pro- 

 fessor E. "W. Morley, reported by J. E. Smith, in the same 

 journal. The measurements were made by means of a 

 Tolles one-sixteenth and a Troughton and Sims micrometer. 

 Professor Morley's measurements are, no doubt, pretty ac- 

 curate, but any one who knows any thing about diatoms 

 also knows that the number of stria3 in 0.01 inch, is subject 

 to considerable variation in the same species. In a com- 

 munication to the Memphis Microscopical Society, he states 

 as a result of his measurements of the striae of Amphipleura 

 pellucida that they number 92,600 to the linear inch. 



The perfection of objectives is yet far from being attain- 

 ed, as we have now Mr. Tolles, with his new system one- 

 tenth surpassing the best work hitherto even with his one- 

 fiftieth ; and Messrs. Powell and Lealand, at a recent soiree 

 of the Royal Microscopical Society exhibited a one-fourth 

 and a one-eighth on a new formula, the first resolving Amphi- 

 pleura pellucida, and the other showing Pleurosigma angu- 

 lation x 4000, under the most difficult test of direct light, 

 in a remarkably magnificent manner, the beads standing out 

 like minute spheres. At the same meeting Messrs. Beck ex- 

 hibited a large microscope in solid silver, fitted with every 

 conceivable piece of apparatus, all in silver. This luxurious 

 w T ork of art, intended for an American microscopist, cost 

 some 500. 



In the Monthly Microscopical Journal for February, 1875, 



