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XXXII. — Observations suggested hy the Study of AmjyJiijileiira 

 peUucida, mounted in Canada Balsam, hy Lamplight and 

 Sunlight, with various Objectives. By J. J. Woodward, 

 Surgeon and Brevet Lieutenant-Colonel U.S. Army, Hon. 

 F.R.M.S. 



iEead lltJi June, 1879.) 



In the latter part of October, 1878, I received from Carl Zeiss, of 

 Jena, two objectives, a | and a yV, both " oil-immersion," or as 

 they are now called by their maker, " objectives for homogeneous 

 immersion." To test their performance, I made with them some 

 photographs of Amjyhijyleura pellucida mounted in Canada balsam, 

 and, for comparison, photographs of the same frustule, with several 

 other objectives. On January 20, 1879, 1 wrote to Zeiss a brief 

 account of these experiments, and sent him a set of the photo- 

 graphs, together with a duplicate set for Professor E. Abbe, of 

 Jena. I learn from the Journal * that Professor Abbe promptly 

 sent one set of these photographs, together with my letter, to 

 Mr. J. W. Stephenson, who exhibited the photographs and read 

 parts of the letter at the meeting of the Koyal Microscopical 

 Society, February 12, 1879. t This circumstance, which was 

 fully in accordance with permission granted in my letter to make 

 public use both of it and the photographs, would render it un- 

 necessary for me to do more at present than to send duplicates of 

 the photographs for the cabinet of the Society, but that, since 

 writing to Zeiss, I have made photographs of the same test with 

 several other notable objectives, especially a i glycerine-immersion 

 by Spencer, and a ^^ oil-immersion by Tolles, and that the results 

 induced me to make a new series of comparative photographs with 

 the best objectives at my disposal, iucludiog, of course, those of 

 Zeiss. This new series I now send to the Society instead of the 

 original one. 



Several matters of interest were first noticed or strikingly 

 illustrated during the progress of the work, and to these it is the 

 object of this paper to draw attention. 



I. The first point was brought out during the original series of 

 experiments, and was detailed in my letter to Zeiss, but does not 

 appear to have been read to the Society, viz, the best mode of pro- 

 jecting images upon the screen for photographic purposes with 

 such objectives as those of Zeiss. These objectives, it will be 

 remembered, have no screw-collar adjustment, and their aberrations 

 are completely corrected only when the focal adjustment is such 

 that the image is formed at a fixed distance, which, in the case 

 of the two objectives sent me, is 10 inches. Now, to obtain the 



* Vol. ii. (April, 1879) p. 2U. t Op. cit., p. 140. 



