ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 417 



an oil-immersion may be used instead, but obviously it must be used 

 with water, not oil ; but this will give a sufficiently good image for our 

 purpose, which is merely to recognize the specimen for marking, not to 

 examine it. If an oil-immersion be not available, any close-working 

 objective, say £ in. or even ^ in. may be used, but it is necessary that 

 the front lens be a small one, so that the spot of water placed by it 

 should be as local as possible. 



There are, of course, some difficulties ; the chief is, that objects 

 mounted in glycerin are somewhat liable to move if at all roughly 

 handled, and may work out of the circle ; but with balsam or glycerin- 

 jelly mounts, or even a shallow glycerin one, there is little danger of 

 this. If a turn-table is not in the outfit of the experimenter, a sufficiently 

 good circle may be drawn by hand, or a line drawn to indicate the posi- 

 tion, or, as has been suggested, the barrel of a mapping pen or similar 

 object may be used. But the first great difficulty is always to indicate 

 the exact spot it is desired to mark, particularly if the object is a very 

 minute one, and that is got over with facility by the method indicated. 



Picking Out and Mounting Diatoms.— The art of selecting and 

 picking out any considerable number of diatoms from a spread which 

 contains a variety of species, and of arranging the selected diatoms 

 successfully for mounting, appears to be attained by comparatively few 

 persons. The methods by which successful results are secured are not 

 generally known. Many workers are able to select a limited number of 

 forms by the hand method of picking, but this method requires great 

 concentration and nerve control. Among the difficulties to be sur- 

 mounted, or at least allowed for, are the pulse beats that affect the 

 hand, the high magnification making the pulsations quite apparent. 



In 1895, J. M. Blake* devised an apparatus which overcomes many 

 of the difficulties. This apparatus is a small pantograph made of light 

 wire. It reduces the motion of the hand twenty-six times. It consists 

 of a jointed parallelogram P made of No. 20 iron wire. This wire is 

 flattened at the points where the rivets are placed. This parallelogram 

 measures 5 in. on each side. Within this area, in one of its angles, is 

 built a minute parallelogram twenty-six times smaller than s. This tiny 

 parallelogram measures T \ in. on each side. The apex of the larger 

 outer form P is coincident with that of the small inner member s, and 

 rotates on the same rivet. This rivet also passes through a short bit of 

 wire, and on this wire P can be rotated in its own plane. This rotation 

 gives P a horizontal motion, while the short bit of wire itself rotates 

 in bearings which give an up-and-down motion at right angles to the 

 plane of P ; P itself slips over and around the body of the Microscope. 

 The short bit of wire referred to has its bearings held about £ in. above 

 the stage of the Microscope. This leaves room for a glass slip H in. 

 wide and 6 in. long to move underneath. A piece of sheet-tin is bent 

 so as to form a clip to spring over the farther edge of the Microscope 

 stage, and to this piece of tin the bearings of the short piece of wire 

 are attached by supporting strips. In use the diagonally opposite apex 



* Araer. Journ. Sci., xxxvii. (1914) pp. 535-8. 



