196 AMPHIPLEURA PELLUCIDA. 
As to the result I may say that I have not yet met an immer- 
sion lens by any first class American, English, or Continental 
maker of an actual focal length of 4 inch or less which did not, 
in my hands, resolve the Amphipleura more or less satisfactorily. 
I have even succeeded with an immersion + of Mr. Tolles’ 
obtaining a good photographic negative of two frustules well 
resolved with only two hundred and fifty-six diameters. My 
friend, Dr. J. W. S. Arnold of New York, writes me that he has 
obtained resolution by a Wales’s immersion }. 
A great difference exists, however, as to the manner in which 
different objectives, even when of the same power and by the same 
maker, will exhibit them, and for myself I have obtained the best 
results only with the finest immersion glasses of Messrs. Wales, 
Tolles, and Powell and Lealand. Spencer’s recent objectives I 
have had as yet no opportunity of trying. With dry objectives 
the task is more difficult, still I have succeeded tolera bly with 
some of the dry objectives of the above makers, and it was a — 
seen with dry objectives that Messrs. Sollitt and Harrison first 
observed the striæ, though they could have glimpsed them but 
imperfectly or they would not have set them down at one hundred 
and thirty thousand to the inch. : 
In illustration of the appearances which ought to be obtained i 
by a first class immersion objective of adequate power, the liie 
ality of the Editors of the NATURALIST enables me to present ge - 
with a Woodbury print from a negative, representing portions of “ 
two Amphipleura frustules as seen by an immersion objective 
Mr. Wm. Wales of Fort Lee, New Jersey, magnified i 
en omr ce 
The objective used was made by Mr. Wales nearly three pe ‘ 
ago. It was named a jj; but is in fact a lower power. On , a 
urement I obtained the following data.’ Magnifying ee ae 
without eyepiece, at fifty inches distance from micro es a 
screen eight hundred and ninety diameters uncovered, tWS e 
hundred and fifty diameters at full correction for cover’ ve - 
of aperture at uncovered 110°, at full cover 130°; at uncor > 
the objective is, therefore, y very nearly. Bey 
