670 Transactions of the Society. 



have remounted it in Canada balsam. Of course the difficulty of 

 resolving so delicate an object as AmphipJeura pellucida is some- 

 what increased by mounting it in Canada balsam. In January, 

 1 852, Professor Bailey, of West Point, writing to Matthew Marshall, 

 Esq., of the surprising performance of the wide-angled dry lenses 

 just constructed for him by Charles Spencer, declared, " In all 

 these cases (and, in fact, whenever I allude to a test object) I mean 

 the balsam - mounted specimens. The dry shells I never use as 

 tests." It is amusing at the present day to note how " extraordinary " 

 this assertion then appeared to one of the most distinguished 

 English microscopists, who contrived then, as he has more than 

 once subsequently done, to misunderstand completely the reasoning 

 of the " American opticians," and who boldly wrote, " I have 

 invariably found that when very difficult tests are mounted in 

 balsam I cannot discover the markings." * 



At the present day the value of difficult test objects mounted in 

 Canada balsam is more correctly appreciated ; and among these one 

 of the most convenient and useful is Amjyhijyleura pellucida. It is 

 not many years since this diatom, even when mounted dry, was 

 regarded as oue of the most difficult tests. Messrs, Harrison and 

 Sollitt indeed appear to have glimpsed the striae on the dry mount 

 as early as 1854; but can only have glimpsed them, for they 

 estimated the number at 120 to 130 in the tuVo of a^i mch., an 

 estimate to which Mr. Sollitt stoutly adhered as late as 1860.t 

 Most other microscopists were unable to verify these observations. 

 Messrs. Sullivant and Wormley :j: declared that they had " not been 

 able to ' glimpse ' the striae on this diatom." And when Mr. 

 Sullivant sent me the Hull slide he had still been unable to resolve 

 it, as indeed was true of almost all microscopists at that date. 



So far as I have been able to learn, Messrs. Powell and Lealand 

 were the first who succeeded in resolving the dry frustules with 

 sufficient distinctness to get correct notions of the fineness of the 

 markings. We learn by a note from Mr. Lobb, dated January 12, 

 1870, § that those gentlemen had at that time succeeded in resolving 

 Amphij^Uura pelhicida with their immersion |, ^l^ and iV, and es- 

 timated them at 100 to the roVcr of an inch. I myself first succeeded 

 in resolving and photographing this diatom in January, 1871, and 

 it was a Powell and Lealand immersion yV that did the work. In a 

 memorandum published by the Surgeon-General's Office, February 1, 

 1871 (and republished in the 'American Journal of Science and 

 Arts,' vol. I, (1871) p. 345), I stated that I found the striae on 

 medium-sized frustules counted from 90 to 93 to the xoVij of an 



* See ' Quarterly Journal of Microscopical Science,' vol. ii. (1854) p. 214. 

 t Ibid., vol. viii. (1860) p. 48. 



j " On the Measurement of the Strinc of Diatoms," the ' American Journal 

 of Science and Arts,' vol. xxvii. (1859) p. 250. 



§ 'Monthly Microscopical Journal,' vol. iii. (1870) p. 104. 



