Gregory, on Diatomaceous Earth. 
99 
form in the Mull deposit. They are represented with 
a power of 400 diameters. I 
find the length to vary from 
1-470 to 1-600 of an inch. 
There are, as Mr. Smith first 
ascertained, 44 striae in 1-1000 
of an inch. It always exhibits 
the two notches towards the 
ends of the plane or slightly 
concave side. Fig. 1 is by far 
the most usual form ; fig. 2 
is, however, not unfrequent. 
The form is very abundant in 
the Mull deposit, and I have only seen it in one other, 
also from Scotland, namely in a slide labelled " From the 
banks of the Spey," which, I had from Mr. Topping. 
Himantidium Arcus, which, when small, has some slight 
resemblance to the above form, has only 22 striae in 1-1000 of 
an inch and its striae are consequently, cceteris paribus., quite 
easily seen, when those of the doubtful form cannot be made 
out. Mr. Smith thinks its place must be near Eunotia Arcus, 
}^xiitzmg=Navicula Arms, Ehr. ; but that it cannot be referred 
to that species. Indeed it is only very immature specimens 
of E. Arcus (Kiitz.) that at all resemble this form, since 
the mature E. Arcus (Kiitz.) has a bend or rounded angle in 
the middle. The doubtful form may be an immature one, 
but what is its aspect when mature ? 
On the Binocular Microscope, and on Stereoscopic Pictures of 
Microscopic Objects. By Professor C. Wheatstone, F.R.S. 
Communicated by Dr. Lankester, F.R.S. (Read April 27, 
1853.) 
In Section 11 of my first Memoir on Binocular Vision, pub- 
lished in the Philosophical Transactions for 1838, I have 
alluded to the illusions to which microscopic observers are 
liable, from their inability to judge correctly the relief of 
objects when one eye only is employed. This inde termination 
of the judgment exists whenever a shadowless object is re- 
garded with a single eye. Frequently an elevation appears as 
a depression, a cameo as an intaglio, a hollow pyramid (as a 
crystal of muriate of soda) as a pyramid in relief, &c., and 
vice versa ; but this indecision is entirely removed when the 
object is viewed with both eyes simultaneously. No mistake, 
if the object be a near one, can then be made with regard to 
