Observations on Amijhipleura ijellucida. By J. J. Woodward. 671 



inch. I found no example in which the number of striae exceeded 

 100 to the T(jVo of an inch. Since then I have examined a con- 

 siderable number of Ampliiijleura slides, and only occasionally 

 found one still finer. The finest I have ever counted had 107 strife 

 to the T^V(T of a,n inch. Ever since January, 1871, Amphipleura 

 jyelhicida has been one of my favourite test objects for immersion 

 objectives. In June, 1871, 1 succeeded in resolving it by an 

 immersion ^ of Tolles,* and in March, 1872, in connection with 

 its resolution by certain objectives made by E. and J. Beck, and by 

 William Wale, I wrote : — " This diatom is a useful and valuable 

 test for immersion objectives of |- inch focal length or less. Lower 

 powers can only hope to resolve it if possessed of excessive angular 

 aperture." f 



Meanwhile, although I constantly wrote and spoke of the dry 

 frustule, because it gave more brilliant images as I then handled 

 the test, I had already, in January, 1871, resolved with the 

 immersion y^ of Powell and Lealand, several balsam-mounted slides 

 of the same diatom, which Mr. Sullivant had kindly sent me. In 

 February, 1871, Count Castracane wrote from Eome to the Eoyal 

 Microscopical Society, t that the year before he had succeeded in 

 obtaining a jDhotograph of Amjjhijyleura ])ellucida on a Moller's 

 test-plate, balsam mounted, first with a Hartnack immersion No. 10, 

 and afterwards with an objective of the same number by Nachet. 

 The degree of success attained by this distinguished microsopist 

 may be inferred from his own frank acknowledgment : — " Un- 

 fortunately my negative was blurred and rather faintish, so that it 

 could not give good positive images. Nevertheless, the stri^ are 

 there so finely and so distinctly drawn out, that they may be 

 perceived clearly enough, though the magnifying power of the 

 Microscope was not higher than 640 diameters." § I myself in 

 March, 1872, wrote in a general way of the resolution of the 

 balsam-mounts : — " I may add that any of these objectives, in- 

 cluding the Beck's tV) will resolve Amphiple^ira ])ellucida in 

 balsam, as in fact was done by Count Castracane, with objectives by 

 Hartnack and Nachet." || 



But all this was by monochromatic sunlight, and it was not 

 until I began to use immersion illuminators with objectives of more 

 than 82^ balsam angle, that I succeeded in obtaining satisfactory 

 resolution of the balsam-mounted frustules illuminated with an 

 ordinary coal-oil lamp. Even then it was by no means with every 

 immersion objective of sufficient aperture that satisfactory resolution 

 could be obtained; but during the last six years the number of 



* See 'Monthly Microscopical Journal,' vol. vi. (1S71) p. 150. 

 t Same Journal, vol. vii. (1872) p. 166. 



% Same Journal, vol. v. (1871) p. 176. § Loc. cit. 



Same Journal, vol. vii. (1872) p. 233. 



2 Y 2 



