On Amphipleura pellucida as a Test- Object. 423 



LII. — On Amphipleura pellucida and Surirella gemma as 

 Test-ohjects. By J. J. Woodward, Assistant Surgeon, 

 U. S. Army*. 



The attention of microscopists has frequently been directed, 

 of late years, to the Aynijldiileura pellucida or Navicula acus 

 as a test-object well suited to try the defining-powers of the 

 very best object-glasses. The length of this diatom is stated by 

 Pritchard as ranging from -^-^ to -^^ of an inch. The average 

 length is given in the ' Micrographic Dictionary ' at '0044 of an 

 inch. The strife, which are exceedingly difficult, were first 

 described by Messrs. Sollitt and Harrison, who estimated them 

 at from 120,000 to 130,000 to the inch. Their estimate has 

 been adopted in the ' Micrographic Dictionary ' and by the 

 majority of modern writers who have referred to this test ; 

 but so many difficulties beset the resolution, that few micro- 

 scopists appear to have attempted to verify the original esti- 

 mates. Indeed most observers would seem to have been un- 

 successful in their efforts to resolve the Amphipleura even 

 with the best objectives; and some have gone so far as to deny 

 the existence of any stria3 upon the frustules of this species. 



Among the microscopists who claim to have seen the strige, 

 several would seem to differ from the original estimates of 

 Sollitt and Harrison as to their fineness. Dr. Eoyston-Pigott, 

 whose papers on " high-power definition," in the ' Monthly 

 Microscopical Journal,' have recently attracted much attention, 

 sets down their number at 150,000 to the inch. Dr. Carpenter, 

 on the other hand, in the 4th edition of ' The Microscope and 

 its Revelations,' expresses the of)inion 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. Lealand has recently " succeeded in counting the 

 Amphipleura-\u\t?>j and finds them 100 in i-oVo of ^^ inch." 



A few months ago two slides oi Amphipleura pellucida were 

 received at the Army Medical Museum from Messrs. Powell 

 and Lealand, and I succeeded in obtaining excellent resolution 

 by the immersion -^-^ of these makers. The frustules on the 

 two slides were found to measure from -j-f-o to -^\^ of an inch 

 in length. Resolution could be satisfactorily effected and the 

 string counted on any of them. I took eight successful nega- 

 tives from medium-sized and small frustules, and verified the 

 counts made in the microscope by counting the strias on the 

 glass negatives. I found the striae on medium-sized frustules, 

 say -;j-^ of an inch in length, counted usually from 90 to 93 striae 

 to the ToVo- of ^^ inch ; in that selected for the two photographs 



* From ' Silliman's American Journal,' May 1871. 



