158 MICROSCOPIC OBJECTS. 



be arranged in shallow drawers, like transparent objects, 

 as hereinafter mentioned. When the objects are very 



minute, and the magnifying powers high, the objects may 

 . be mounted on the heads of common pins ; but remem- 

 ber that this method should not be adopted when the 

 aperture of the magnifier is larger than the head of the 

 pin, as the direct hght from the condenser would be 

 admitted and produce glare. 



(7.) Method of viewing the Internal Organization of 

 Animalcules. — The usual food of these animals assimilates 

 so closely m colour to themselves, that it is impossible 

 under ordinary circumstances to perceive the form of 

 their digestive functions. During the investigation of 

 the polype by Mr. Trembley, he endeavoured to ascertain 

 whether the small granular bodies dispersed over its sur- 

 face were digestive cavities, and for this purpose fed these 

 creatures on organic coloured substances, such as indigo. 

 This idea has been recently followed up by Dr. Ehren- 

 berg of Berlin, who has successfully employed minutely- 

 divided coloured substances, such as indigo, carmine, and 

 sap green, for ascertaining the form of the digestive ca\a- 

 ties in animalcules. It is essential that whatever the 

 colouring-matter we employ be, it must be pure and free 

 from any metallic impurities, and that it be only mecha- 

 nically, not chemically soluble in water. For more minute 

 directions on this subject see ' General History of 

 Animalcules.' 



