100 SELECTION AND USE 



microscopists to construct them for themselves. Where this is 

 desirable, however, the best method of making them is as 

 follows: Select a piece of plate glass, of the thickness of an 

 ordinary slide, and cut it about three inches by one and a quar- 

 ter. Then select another piece of glass, as thick as the trough 

 is to be deep (from front to back), and cut it to the size of the 

 outside of the trough. From the bottom of this piece of glass 

 cut a strip a quarter of an inch -wide, and from the sides cut 

 also strips of the same width. The centre piece may now be 

 thrown aside, and the ends of the side strips will make a tight 

 joint with the bottom strip. The three strips should then be 

 cemented to the large plate, and over them should be cemented 

 a piece of the thin glass used for covers^ The strongest cement 

 is marine glue, but we are told by Prof. Starr, who is well 

 known for his success in keeping and exhibiting living micro- 

 scopic objects, that marine glue is very apt to destroy ani- 

 mal life. He, therefore, uses old Canada balsam, and we have 

 seen a large variety of minute forms of animal and vegetable 

 life which had been kept for months in a healthy condition in 

 such troughs or cages. 



Compressorium. In the examination of certain objects, 

 it is frequently necessary to flatten, and even to crush them, 

 in order to render their structure visible. This is best accom- 

 plished by means of a well-made compressorium, of which 

 there are several different kinds in use. The ordinary com- 

 pressorium consists of a metal plate, in the centre of which is 

 fixed the disc of glass upon which the object is laid. A second 

 disc, fastened in a ring which is hung at the end of a lever, by 

 means of two pivots, is pressed against the first by means of a 

 screw, which tilts the lever. In this way a very strong pressure 

 may be exerted, while, owing to the free movement of the ring on 

 the pivots, the plates of glass always remain parallel to each other. 



A compressorium which may be used to examine either side 

 of an object is constructed by Messrs. Ross & Co. In this form 

 the two plates are brought together by means of a wedge, 

 which is moved by' a screw. 



Gravity Compressorium. The animalcule cage above 

 described was invented by Varley, many years ago, and is very 



