NOTES AND MEMORANDA. 
217 
of the separated cells of Bamhusina, probably from the filament it had 
swallowed in the afternoon. 
These observations apparently explained certain facts in the habits 
of the animal. Dinamoeba had been noticed to be especially fond of 
the Alga Didymoprium, for it was found to be present as the principal 
element of the food in numerous specimens. Bambusina was less 
frequently found among the food contents of the animal. The Alg8B 
were equally abundant in the localities of the Dinamoeba, and from the 
observations detailed it would appear that the Didymoprium is pre- 
ferred as food from the comparative ease with which its filaments are 
broken into pieces of convenient size for swallowing. 
The observations are, moreover, interesting from their indicating 
discrimination and purpose in the movements of one of the simplest 
forms of animal life. The movements are to be viewed as reflex in 
character, though resembling the voluntary movements by which 
the most intelligent animal would prepare morsels of food of con- 
venient form to take into the mouth. In striking contrast were the 
movements, noticed on several occasions, by which an Oscillatoria 
obtained entrance into the empty shell of an Arcella, and there, coiled 
up, crept round and round incessantly. 
On the Measurement of the Dihedral Angles of Microscopic Crystals . — 
M. Em. Bertrand has communicated the following to the French 
Academy : — 
The goniometer of Wollaston, more or less improved, is the only 
apparatus now employed for the exact measurement of the dihedral 
angles of crystals, and with this instrument in its present condition 
very small crystals can be measured. There is, however, a limit beyond 
which it becomes insufficient, and a crystal whose sides are for instance 
only the thirtieth of a millimetre cannot be measured by means of it. 
A method which would allow the dihedral angles of microscopic 
crystals to be measured, presents therefore some interest, for the 
crystals are generally purer the smaller they are. 
For this I have endeavoured to make use of the microscope, but 
the difficulty which was at once presented was the orientation* of 
the crystal to be measured. By means of the procedure explained 
below, this orientation becomes useless, and we are able by an indirect 
process to calculate the angle of the two faces of a crystal without it 
being necessary to orient it. 
Take a cube and place a crystal in any position on one of the 
faces of it. Suppose one of the faces of the crystal to be prolonged to 
meet the face of the cube on which it is placed, the projection of this 
face of the crystal on the face of the cube will form with two of the 
edges of the cube two plane complementary angles. If we suppose 
this face of the crystal prolonged beyond the face of the cube on which 
it is placed, we shall obtain upon two other faces of the cube two 
projections, making respectively, with two edges of the cube, plane com- 
plementary angles, and the direction of the face of the crystal will be 
* This word may well be adopted into the English vocabulary. 
VOL. I. E 
