ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
515 
reference to these tables, the data referring to 1*00 were found to be 
Nitzschia curvula and Navicula rhomboides ( Frustulia ) var. saxonica 36 
striae in 1/100 mm., while for the resolution of Amphipleura pellucida 
with 40-42 striae, a system of 1* 10-1 *15 was found to be necessary. 
Long before the use of the apertometric process, Herr Kayser had 
informed the maker of the system that he fixed the resolving power at 
34 striae from the fact that Nitzschia curvula was not resolved, and that 
Frustulia showed striae first on the edges. The maker, ascribing the 
non-resolution to the mounting of the preparation, at the same time sent 
preparations which really were resolved. The striation of these, how- 
ever, only amounted to 26 and 21 to 30 respectively, while Amphipleura 
pellucida was not forthcoming, because they were “at present not of 
good quality.” 
Dippel’s work shows with what exactness the productions of micro- 
scopical forms can be apertometrically rated, in a way quite analogous 
to the determination of size by the scale. The action of an optician 
therefore who sells an objective system having a less aperture than it 
professes to have, must be compared to the behaviour of a tradesman 
who supplies goods deficient in quantity. 
An advantage is now to be considered which the apertometer ring 
method possesses over that of Abbe. In the latter method a pointer 
is turned round on a polished glass cylinder until it appears to come 
on to the edge of the aperture. In this way the aperture is tested 
only in a certain diameter. By the author’s method the whole range is 
seen at a glance, and any defects can also be noted. It is interesting 
that in the present dry system No. 7, the rings do not appear to be 
exactly concentric, but in a certain diametral direction on one edge 
there are broad intervals, on the opposite narrow ones, so that for the 
clear definition of the first, a further pressing in of the eye-piece is 
necessary. This asymmetry can be also recognized by the first method 
in the change in adjustment of the eye-piece, and out of the difference 
of the horizontal angle. 
Plaxton, J. W. — A Camera Lucida for nothing. 
[“ The other day, after a morning’s work, something went wrong with the 
prism of my camera lucida, and, do what I would, I could not bring it back 
to usefulness. At a loss for the moment, I cast about 
for a substitute, and in half-an-hour, with penknife 
and pencil, out of a piece of stiff paper and a square 
of thin glass, had turned out a fragile but efficient 
substitute for what is known in catalogues as ‘ Beale’s 
Neutral Glass Reflector,’ price 6s. 
“This is how I did it — Describe a circle by standing 
the eyepiece of the Microscope on the paper and 
running a pencil round it ; inscribe a square in the 
circle already drawn by drawing the pencil along 
the edges of the square of thin glass you intend to 
use ; now lay down the diagonals of the square ; 
draw three other lines within the square, each one 
parallel with a side of the square, and each, say 
1/8 inch from the side ; draw two other short 
lines (a a in the diagram, fig. 57) parallel to the 
diagonals. 
Take the penknife and, following the continuous lines of the diagram, cut 
through the paper : you will have in paper what resembles a three-spoked 
Fig. 57. 
