3o4 PREPARATION AND MOUNTING 
upon the slides and thin glass upon the cell are thus kept 
uniform. Dr. Matthews, a gentleman of no little experience, 
has given us an improvement as follows :—Take two “jaws” 
of the average thickness of a glass slide, ¢ inch wide, 25 
long. Each of these is pivoted on the face of the turntable . 
by a screw through its centre, each screw being placed . 
exactly equidistant from the centre of the turntable, so that - 
the jaws are separated by a space as wide as an average 
slide; i.e. a full inch. Outside of that space, on one side of 
the centre of one of the jaws, is a wedge fixed by a screw in’ 
such a way as to be capable of motion in the direction of 
its length by a slotted hole. This is all the machinery. 
AB and CD are the two jaws, E is the wedge. On placing 
a slip between the jaws they probably at first do not touch 
it. If the wedge be then pushed so as to approximate B to 
C, the jaws move on their centres, so that, however far B 
may be pushed towards (and moving) C, the other end of 
C—i.e. D—is moved eaactly as much in the opposite direc- 
tion until they approach near enough to grasp the slide by 
its edges. The length of the wedge must, of course, be such 
as to provide for about 4 inch variation in the width of 
slides. It will readily be seen that the slip may be pushed | 
in either direction excentrically lengthwise, so as to allow of , 
the formation of any number of cells, all of which must 
needs be central as regards their width, if the instrument 
has been accurately made, which is a very easy matter. I 
have added also a rest for the hand, F, which may be turned 
aside on a centre at will, and which I have found to be a 
great convenience. 
