44 PROGRESS OF MICROSCOPICAL SCIENCE. 



recently attracted much attention, sets down their number at 150,000 

 to the inch. Dr. Carpenter, on the other hand, in the fourth edition 

 of ' The Microscope and its Eevelations,' expresses the opinion that 

 even the estimates of Messrs. Sollitt and Harrison are too high : and 

 we are told by Mr. Lobb (' Monthly Microscopical Journal,' vol. iii., 

 p, 104) that Mr. Lealaud has recently "succeeded in counting the 

 Amphiplcura lines and finds them 100 in -r^th of an inch. A few 

 months a^ o two slides of Amjihipleura pellucida were received at the 

 Ai-my Medical Museum from Messrs. Powell and Lealand, and he 

 succeeded in obtaining excellent resolution by the immersion y^^li of 

 these makers. The frustules on the two slides were found to measure 

 from j-yTjth to ^^th of an inch in length. Eesolution could be satis- 

 factorily effected and the striae counted on any of them. He took eight 

 successful negatives from medium size and small frustules, and veri- 

 fied the counts made in the microscope by counting the strife on the 

 glass negatives. He foimd the strife on medimn-sized frustules, say 

 giyth of an inch in length, counted usually from 90 to 93 stride to 

 the ^o^Qp th of an inch; in that selected for the two photographs 

 which were sent to the editors, the number was 91 to the yoW*^ 

 of an inch. Larger fi'ustules exhibited rather coarser, smaller ones 

 rather finer striae. On the smallest frustules at his disposal, several 

 of them only 4^th of an inch in length, he found no example in which 

 the number of strife exceeded 100 to the y^Voth of an inch. The 

 strife of these smallest and most difficult frustules do not then rival in 

 fineness the nineteenth band of the Nobert's plate, as has been 

 asserted by some ; they compare rather with the sixteenth and seven- 

 teenth bands. After making the photographs, he extended his obser- 

 vations to a number of other slides of Ampldpleura j)ellucida, including 

 two of the original specunens from Hull, kindly sent to the Museum 

 some time since by Mr. W. S. Sullivant, of Columbus, Ohio, and the 

 example in the First Century of Eulenstein. He found that different 

 slides varied considerably in the ease with Avhich he could resolve 

 them, chiefly as he thinks on account of the thickness of the glass 

 covers, which in several instances did not jjermit the best work of the 

 immersion yV*^- Perhaps, however, the markings on some frustules 

 may be shallower than on others whose strife count the same number 

 to the yJy-(jth of an inch. In any event he has found, as yet, no 

 slides the covers of which permit the y^th to be approximately 

 adjusted, on which it was impossible to resolve the frustules, and no 

 frustules the striae of which exceeded 100 to the yoV^t^^ ^^ ^^ inch. 

 The best resolution he was able to obtain by ordinary lamplight was 

 not very satisfactory. He used therefore, dm-ing the investigation, 

 direct sunlight, rendered monochromatic by passage through the solu- 

 tion of ammonio-sulphate of copper. A parallel pencil of such light 

 was concentrated by the achromatic condenser, which was suitably 

 decentred to attain obliquity. The same illumination was employed 

 in making the photographs. He has since had the pleasm^e of exhibit- 

 ing the resolution in quite as satisfactory a manner to several micro- 

 scopists by monochromatic light obtained from the electric lamp. 



