WITH THE MICROSCOPE. 155 



with a pipette, or upon a glass rod, or with the finger, and placed upon 

 the glass slide. A bristle or thin piece of paper is placed in such 

 a position as to prevent the thin glass from coming into too close 

 contact with the slide ; or the drop may be placed in a Brunswick 

 black, or thin glass cell ; or the animalcule cage already described, 

 pi. XVIII, fig. 112, may be used with advantage. By the latter instru- 

 ment, and also by the compressorium, p. Si, the larger infusoria may 

 be kept still in a particular position for the purposes of examination. 



Fresh-water and marine zoophytes, too large to be placed in the 

 small cells, or the troughs, p. 66, may be examined in flat watch 

 glasses, or in one of the larger cells, represented in pi. XVIII. These 

 may be examined with low powers (two inch, one inch) without any 

 thin glass cover, but where the higher powers are employed a piece 

 of thin glass must be applied in such a manner as to cover that part 

 of the vessel in which the animals are situated, while at the same time 

 a certain proportion of the fluid is freely exposed to the air ; for if 

 aeration were prevented, the animals would soon exhaust all the air 

 dissolved in the small quantity of water in which they were impri- 

 soned, and die of suffocation. 



It is difficult to kill many zoophytes, and preserve them with the 

 tentacles extended, but it is said that the retraction of the tentacles 

 may be prevented by plunging them into cold fresh water. Various 

 poisons, opium, hydrocyanic acid, chloroform, &c., have also been 

 recommended for the same purpose, but a voltaic current effects the 

 object most perfectly. 



Vorticellce and Rotifers or wheel animalcules, may often be 

 obtained by placing a small piece of a plant which has been allowed 

 to remain in the same water for some time, with a drop of the fluid 

 in a glass cell. These organisms are often found attached to the 

 edges of the plant in considerable number. 



Cheese mites and other small acari should be examined with low 

 powers (two inch, one inch) under the binocular, a strong light 

 being condensed upon them with the aid of the bull's eye condenser 

 or the parabolic reflector, p. 21. 



The Entozoon or Demodex folliculorum may be obtained by 

 squeezing the sebaceous glands in the skin of the nose or scalp of 

 the human subject. If transferred to a little oil and covered with thin 

 glass the animals may be preserved alive for some time and their 

 slow sluggish movements watched. Specimens may generally be 

 obtained from the wax of the ear. Small spiders, many of the minute 

 coleoptera and their larvns, aphides, and a great number of the 

 smallest insects common in fern cases may be easily examined, and 

 some of the smallest of them are suitable for examination by reflected 



