1G6 Transactions of the 



to "wliicli I referred in the ' Montlily Microscopical Jonrnal ' for 

 December, 1869, p. 292, as resolving the seventeenth band of 

 the Nobert's nineteen-band plate. The picture of Amphipleura 

 obtained exceeds all I have been able to do on this diatom with any 

 objective except the immersion -j^g^th (so-called) of Messrs. Powell 

 and Lealand. I forward a print herewith showing a pair of 

 frustules, both handsomely resolved from end to end, with about 

 1500 diameters. For comparison I send a picture of the same 

 frustules taken with 1650 diameters by the Powell and Lealand 

 ^th. 



The Wales's objective was invoiced to the Museum as a ^V^'^- 

 A determination of its magnifying power, however, shows that at 

 uncovered it magnifies rather less, at covered rather more, than the 

 Powell and Lealand objective. I get by actual measurement 

 the following results : — At 50 inches distance from micrometer to 

 screen this objective magnifies 890 diameters uncovered, 1250 

 covered. Its equivalent focal length at uncovered is therefore ^^yth 

 very nearly. 



In conclusion, I may add a few words with regard to the use of 

 Amphipleura 2>^il^ucida as a test-object. This diatom is a useful 

 and valuable test for immersion objectives of ith inch focal length 

 or less. Lower powers can only hope to resolve it if possessed of 

 excessive angular aperture. When, however, it is desired to 

 discriminate between small difi'erences in the excellence of objectives 

 intended for the most exquisite resolution, a more subtle test is 

 required, and this will be found in the nineteen-band plate of 

 Nobert, by those who take j)roper precautions in its use. Those, 

 however, who believe they have secured resolution whenever they 

 see lines in the higher bands of the plate, without duly considering 

 their number, must not be surprised if objectives they have accepted 

 as resolving the ultimate bands of the plate fail to show the strias 

 on even the coarsest frustules of the Amphijjleura pellucida. 



