s 
194 AMPHIPLEURA PELLUCIDA. 
but finer ones in none. For the best use of the test it is essential 
that the frustules should be clean and mounted dry,* on the under — 
surface of a very thin cover (not thicker than z4, of an inch). 
In some of my slides the frustules are mounted between two thin 
covers, adhering to the upper one but I am not sure bos this 
arrangement offers any decided advantages. 
The first step in the practical use of this test, after obtaining 
a properly mounted specimen, is to select a frustule, to count the 
number of its striæ to the thousandth of an inch, and to ta 
its position with a Maltwood’s finder. 
_ The frustule thus selected becomes a valuable unit of compar- 
ison between different objectives, the distinctness with which the : 
stri are shown indicating the definition of the glass, the man- 
ner in which the edges of the frustule are seen while the mid-ib 
and striæ are in focus showing the degree of penetration, and the 
appearance of the ends of the frustule when the centre is in focus 
giving a fair idea of the flatness of the field. 
The illumination must be oblique, and the pencil of light most 
be thrown lengthwise along the frustule, which may be done by 
a common coal-oil lamp, with or without a small plano-comv® 
lens, or other condensing apparatus, to concentrate the rays. yee 
however, is the least favorable mode of illumination, and will on 
succeed if very carefully used with the best objectives. 
better are the calcium and magnesium lamps, which may be 
densed obliquely by means of a small plano-convex lens of one 
three inches focal length. Either source of light gives & w 
ful picture, the striæ being black on a white ground. + 
*This is essential to the best and most beautiful appearance; it i 
dispensable for re tery nor does balsam mounting make resolu 
difficult. For example I obtain excellent resolution of the balsam 
om $ 
I desire, also, to draw attention to the fact that Count F. Ca; states hat 
vad: before - ge yal Microscopical Society, March 1, 1871, expressly 
? 
year had made a photograph of the balsam-mounted Am; 
of Moller’ “ag e, obtaining good resolution and counting th 
gives e millimetre. For this purpose he u o. 10 of 
with monoc sunlight obtained b m 
rather faintish, so that it would not give good positive images. 
about 640 d fterwards he obtained the same resul 
(Mon Microscopical nene April, 1871, p- 176). I may 
No. 11 belonging to m (price 250 francs) gives exce 
e of ass pen and of other balsam-mounted § 
to our collection 
