156 MICROSCOPIC OBJECTS. 



completed.* As their size need not exceed that of the 

 diagram, several of these sliders with test- objects may 

 be carried in a pocket-book, and are always ready to 

 examine the merits of any instrument that may present 

 itself. 



(4 . ) On Mounting Transparent Aquatic Objects and Dis- 

 sections. — ^]Many small subjects of natural history are so 

 delicate that when dried their parts are shrivelled so much 

 that it is with difficulty their features are recognised. It 

 therefore becomes important to discover some method to 

 preserve as, much as possible their beauty, colouring and 

 lineament. This is best accomplished by placing the 

 object on a slip of glass, and covering it with a piece of 

 thin glass, interposing a drop of a thick solution of gum 

 or isinglass ; by this means the object is prevented from 

 drying, and when the gum has hardened, is effectually 

 preserved. In this way may be mounted all the aquatic 

 objects described in this work,t many of wliich cannot 

 be preserved in any other way ; they are the nearest 

 approach to living subjects I have seen. These sliders 

 should have a piece of paper pasted over the thin glass 

 to protect it from injury, leaving an aperture in the centre 

 for the object and made of an uniform size. This method 

 of mounting objects led to the subsequent improved one 

 of Canada balsam ; indeed the substitution of balsam 



* If thought necessary, the edges may be cemented ; if the hole 

 is small, it acts as a stop or diaphragm, and a known object is also 

 thus more easily found. 



t See Notes on Natural History. 



