EXAMINATION OF MARINE OBJECTS 93 



provide a miniature dissecting trough as an accessory to the dissecting 

 microscope, and the following instructions will enable the reader to 

 construct a highly satisfactory and inexpensive one : 



Procure the flat lid of a cylindrical tin box, or the lid of a glass 

 or porcelain pomade pot, such lid to be about two inches in 

 diameter and about half an inch in depth. Cement the flat side of 

 this lid to a small slab of hard wood, or to a square piece of sheet 

 lead, by means of acetic glue ordinary glue or gelatine dissolved 

 in glacial acetic acid to give it the necessary steadiness during 

 the dissection. When the cement is quite hard, pour into the lid 

 some melted paraffin (paraffin wax) which has been blackened by 

 the admixture of a small quantity of lamp-black in the form of a 

 fine powder. The paraffin should be melted by putting it into a 

 beaker or wide-mouthed bottle, and standing it in hot water, and 



FIG. 46. A SMALL DISSECTING THOUGH 



the lamp-black should be added, with stirring, as soon as it is 

 entirely liquefied. The quantity of the mixture used must be 

 sufficient to half fill the lid, thus leaving a space to contain water 

 to the depth of about a quarter of an inch. The blackened wax 

 provides a good background on which to work, and provides a 

 hold for pins when these are necessary in order to fix the object 

 under examination. 



The complete trough is represented in fig. 46 ; and will be 

 found to answer its purpose admirably, except that it occasionally 

 displays one fault, but one that is easily remedied. The wax 

 contracts on cooling, and may, therefore, detach itself from the 

 trough ; and, being lighter than water, will float instead of re- 

 maining submerged. This may be prevented by securing the disc 

 of wax in its place by means of a ring of brass wire, or by weighting 



