4 
I. ROBINSON— THE DIATOMACE2E. 
in length or diameter. Now it is very easy to speak of these 
small dimensions, but by no means so easy to realize them. 
An illustration may however help us to do so. Let us take a 
specimen of Surirella minuta, which, though measuring only one- 
thousandth of an inch in length, is most distinctly and regularly 
marked round its margin. If we measure the thickness of an ordi¬ 
nary sheet of note paper, we shall find that it is equal to about 
= 2 iroth of an inch. If therefore we place four specimens of Surirella 
minuta in a row, the length of that row will exactly equal the 
thickness of the note paper. 
Lor the sake of a practical illustration of the exceeding minute¬ 
ness of these forms, I have mounted some of them upon a slide 
together with a very small needle, of the size known to ladies as 
No. 10. When placed under the microscope several hundreds of 
diatoms, comprising many different species, may he seen within the 
eye of that needle. 
The number of frustules which may he contained in a single drop 
of water is, as may he imagined, very great. I have a slide of 
Admanches subsessilis mounted from specimens which I found in 
my own garden at Hertford, and which - comprises the contents of 
one single drop only, but which a careful estimate has shown to 
number upwards of 200,000 separate frustules. Another mount, 
of Admanches exilis, a smaller form which I found in a ditch by 
the railway between Hertford and Ware, prepared in the same way 
and comprising the contents of only one drop, afforded a still more 
astonishing result. In this case the number of frustules contained 
in the one drop was found to be about 430,000. In each of these 
instances the diatoms occupied but a fraction of the entire bulk of 
the respective drops, probably not more than - 2 -Vth part. I arrive 
at this conclusion because I find that in the small phials in which 
these specimens are preserved, and which contain a depth of about 
inches of water, the diatoms when allowed to settle form only a 
thin stratum of from -r§-th to Atrth of an inch in thickness. The 
total bulk of a drop of water would therefore in the former instance 
represent the aggregate size of five millions of frustules, and in the 
latter that of upwards of ten millions. 
It will of course be very obvious that in order to obtain a correct 
knowledge of these minute forms, we must make use of the highest 
available powers of the microscope. In prosecuting the study of 
local species I have been accustomed to employ two objectives, one 
of half an inch focus, giving with the “ B ” eye-piece a magnifying 
power of 180 diameters, and the other of -Afth inch focus, giving with 
the same eye-piece a power of 750. I have used these on what is 
termed a nose-piece, and they are so arranged that they are exactly 
interchangeable, so far as focal distance is concerned, and the 
different illumination required by the two powers is easily accom¬ 
plished by means of a Webster’s condenser. This has proved to be 
a most convenient arrangement. 
The magnifying powers just mentioned are, however, in one 
direction only, but as length and breadth are both equally extended, 
