846 J. J. Woodward on the AmpUphura pellncida. 



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

 the two slides were found to measure from xTo'th to ji;;th 

 of an inch in length. Resolution could be satisfactorily effec- 

 ted and the strife counted on any of them. I took eight 

 successful negatives from medium size and small frustules, 

 and verified the counts made in the Microscope by counting 

 the striae on the glass negatives. I found the stri» on medium 

 sized frustules, say ^^ o^h of an inch in length, counted usuallv 

 from 90 to 93 striee" to the y^noth .of an inch; in that selected 

 for the two photographs which accompany this memorandum 

 the number was 91 to the y oV o^h of an inch. Larger frustules 

 exhibited rather coarser, smaller ones rather finer striae. On 

 the smallest frustules at my disposal, several of them only 

 yijth of an inch in length, I found no example in whicli the 

 number of strias exceeded 100 to the yoVoth of an inch. The 

 striae of these smallest and most difficult frustules do not then 

 rival in fineness the nineteenth band of the Nobert's plat 

 has been asserted by some; they compare rather with tm 

 teenth and seventeenth bands. 



After making the photographs I extended my obsen;! 

 to a number of other slides of Amphipleura pellucida uku.- 

 ing two of the original specimens from Hull, kindly sent to the 

 Museum some time since by Mr. W. S. SuUivant, of Columbus. 

 Ohio, and the example in the First Century of Eulenstein. 1 

 found that different slides varied considerably in the ease with 

 which I could resolve them, chieflv as I think on account ot 

 the thickness of the glass covers, which in several instances did 

 not permit the best work of the immersion yV^^^- P^A^ps, 

 however, the markings on some frustules may be shallower 

 than on othere whose striae count the same number to the 

 yoVo^h of an inch. In any event I have found, as yet, do 

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

 mately adjusted, on which it was impossible to resolve t^ 

 frustules, and no frustules the strise of w^hich exceeded IW w 

 the ToV flth of an inch. , 



The best resolution I was able to obtain by ordinary im 

 light was not very- satisfactory. I used therefore, d^^^^^,. 

 investigation, direct sunlight, rendered monochromatic by pa^ 

 sage through the solution of ammonio-sulphate of copper- 

 parallel pencil of such light was concentrated by the achromaj^ 

 condenser, which was suitably decentred to attain ^^^^^^J .;,'. 

 The same illumination was employed in making the f^ 

 graphs. I have since had the pleasure of exhibiting the re. 

 lution in quite as satisfactory a manner to several microscopic 

 by monochromatic light obtained from the electric lamp. 



"War Department, Surgeon General's Office, 

 Army Medical Museum, Feb. 1, 1871. 



