34 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, f inch wide, 2 

 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 

 i.e. D is moved exactly 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 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. 



