J. J, Woodward on the Amphipleura pellucida. 



vhich have affected the valley of the lakes in post-gI:i- 

 e. The field for exploration is large, and the d°at;i 

 ^-ivady accumulated inconsiderable, but we may hope the geo- 

 logical surveys now in progress in Ohio, Indiana and Michigan, 

 and ui Canada, will in a few years develop the whole subject. 



^^'^. H.— Memorandum on the Amphipleura pellucida; by 

 J. J. WooDWAKD, Assistant Surgeon, U. S. Army. 



The attention of microscopists has frequently been directed 

 ot late years, to the Amphipleura pellucida or Navicula acus, 

 a.> a test object well suited to try the defining powers of the 

 erv best object glasses. The length of this diatom is stated 

 ^' i'ntchard as ranging from ^^tb to 3Kth of an inch. 

 •riAif^'f^^®. ®^^'^ is given by the Micrographic Dictionary at 

 0044 of an mch. The striae, which are exceedingly difiicult, 

 ^erehrst described by Messrs. Sollitt and Harrison, who esti- 

 timated them at from 120,000 to 130,000 to the inch. Their 

 stimate has been adopted by the Micrographic Dictionary and 

 tli I "^^Jo^^y of modern writers who have referred to this 

 y, but so many difficulties beset the resolution that few mi- 

 croscopists appear to have attempted to verify the original 

 estimates. Indeed most observers would seem to have been 

 STI^*"^ in their efforts to resolve the Amphipleura even 

 i"^ *^^ '^est objectives and some have gone so far as to denv 

 "« existence of any striae upon the fmstules of this species. ' 

 spvp^?^^ microscopists who claim to have seen the stria, 

 ^I^^!^} ^?^d seem to differ from the original estimates of 

 their fineness. Dr. Eoyston Pigott, 

 definition " in the Monthly Mi- 

 down ti"^^ "^-"^iuiu nave recently attracted much attention, sets 

 thp n+i^ \^ number at 150,000 to the inch. Dr. Carpenter, on 

 Re °i 5 ^^,^d' in the 4th edition of "The Microscope and its 

 Mp Ji! o'^f;. expresses the opinion that even the estimates of 

 Mr T ■ ki^ /? ''''^ Harrison are too high: and we are told by 

 thatM T V^^*^^^ Microscopical Journal, vol. iii, p. 104) 

 DhinU ^" ,^^l^nd has recently " succeeded in counting the Am- 

 i^^^ieura hues and finds them 100 in ^oVoth of an inch." 

 receivS" ^^^'^^^s ago two slides of Amphipleura pellucida were 

 and W 1 ® ^^"^y Medical Museum Irom Messrs. Powell 

 Am Jo ' ^°^ ^ succeeded in obtaining excellent resolution 



OCR. Sci,_Third Series, Vol. I, No. 6.— Mat, 1871. 



