200 



THE MICROSCOPE. 



an operation which requires care and perseverance, as most 

 of the animal tissues are very difficult of separation. All 

 substances should be carefully separated from dust and 

 other impurities which renders their structure indistinct 

 or confusing. With very delicate membranes, and with 

 those of the nervous system of the smaller animals, in- 

 sects, &c., it becomes necessary that the investigation 

 should be carried on under water, or in fluid of some sort, 

 in a glass cell, and having a strong light thrown down 

 upon it by the aid of the condensing lens, as represented 

 in fig. 128. A certain amount of change of structure must 

 be expected and allowed for ; as nearly all membranes 

 imbibe some portion of the fluid. Delicate structures are 

 often advantageously wetted with dilute solutions of sugar 

 or common salt, to prevent the changes from endosmosis, 

 which result from the use of pure water. The contents of 

 bodies are frequently rendered more distinct by the addi- 



Fig. 128. Dissecting under water. 



tion of re-agents. If the object be a portion of an in- 

 jected animal, it is better to pin it out on a leaded-cork, 



