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XXXIII. — Note on Abies Experiment on Pleurosigma angulatum. 



By J. J. Woodward, Surgeon and Brevet Lieutenant-Colonel, 

 U.S. Army, Hon. F.K.M.S. 



{Read llth June, 1879.) 



I SEND herewith two photographs of Pleurosigma angulatum taken 

 by the Zeiss oil-immersion | and a ToUes amplifier. The magni- 

 fying power is 1850 diameters. The one exhibits the markings as 

 ordinarily seen and described ; the other, taken with oblique light 

 and a stop, furnished for the purpose by Zeiss, behind the back 

 lens, exhibits the fine difiraction lines parallel to the midrib, 

 described by Professor Abbe, of Jena. 



The notable point about this second picture is that the difi'rac- 

 tion lines, instead of being limited to the part of the frustule which 

 is adherent to the glass cover, as is the case when Abbe's experi- 

 ment is performed by lamplight, appear by monochromatic sunHght, 

 and of course in the photographs, on all parts of the frustule. Pro- 

 fessor Abbe, who originally fell into the error of supposing the 

 absence of the diffraction lines on the non-adherent parts to be the 

 necessary result of the optical conditions, has pointed out with 

 commendable frankness * that his mistake arose from not having 

 before accurately measured the true distance of these difiraction 

 lines apart ; and has willingly admitted that under the actual con- 

 ditions it is " a matter of intensity of illumination only, whether 

 they will be visible or non- visible through a film of air." I 

 call special attention to his lucid explanation,t because in my 

 opinion the greater share taken by the diffraction pencils in the 

 formation of the microscopical image in wide-angled objectives, 

 when illuminated by monochromatic sunlight, has a great deal to 

 do with the superiority of this mode of illumination over lamplight 

 for the purposes of high-power definition, as well as of photo- 

 micrography. By lamplight these diffraction pencils are so much 

 feebler in intensity, that their share in the formation of the image 

 is considerably less. I have in a previous paper (p. 663) pre- 

 sented other striking illustrations of this important point, and 

 the views there expressed are corroborated by the phenomenon 

 observed when Abbe's experiment is performed by monochromatic 

 sunlight. 



The pair of photographs herewith presented to the Society are 

 not from the negatives of which I sent last January to Professor 

 Abbe the prints exhibited at the meeting of February 12th by Mr. 

 Stephenson. Those were accidentally taken with slighfly different 

 distances, and were hence magnified differently. The present 

 * TLis Journal, vol. ii. (April, 1879), p. Ul. f Loc. cit. 



