APPARATUS FOR HOLDING SPECIMENS 55 



witli a dry object glass, they should be dried upon the cover 

 glass and placed film downwards upon the slip. If an immersion 

 object glass is used they may be dried on to the slip and the 

 use of a coVer glass dispensed with, for the whole space between 

 the object and the lens is filled with what corresponds to glass. 

 The thickness of the cover glass, therefore, makes no difference 

 optically, but imless the object is thoroughly dried a cover glass 

 may be required to prevent the object from floating ofE into the 

 immersion fluid. By putting a drop of Canada balsam between 

 the cover glass and the slip, and firmly pressing them together, a 

 permanent mount may be prepared. 



When objects in a drop of fresh or salt water are placed between Biamination 

 a cover glass and a slip, the superfluous fluid around the cover a^^J'"'' *" 

 glass should be removed with blotting or filter paper, and capfllary 

 attraction will hold the cover glass in position when the slide is 

 placed at an angle. 



If a specimen is to be examined for a long period, a piece of 

 cotton may be placed between the cover glass and the slip, one 

 end of which dips into a bottle or capsule of water at a higher level 

 than the slip, and the other in a similar bottle at a lower level. 

 By this means the slide will be kept moist and objects can be kept 

 alive for a considerable period. 



Small organisms, such as infusoria, bacteria, or protozoa, sup with 

 have sufficient room in the thin layer of water between the cover 

 glass and the slip to live and move 

 freely, but larger objects, such as 

 rotifers, entomostraca, etc., require 

 more room. For use with such 

 objects, slips are made with cavi- 

 ties, and are known as slips with Fiq. 50. — No. 3405. 

 hollows. They are used in the 



Qsame way as ordinary slips, the water which 

 fiUs the cavity holding the cover glass in 

 position by capillary attraction. 

 O^^ Cells or rings of vulcanite metal or glass oeiia. 



j^J may be cemented to 3 X 1-inch slips with 

 jBi g^ ^ Hollis glue, forming deeper cavities for the 



^3 OO reception of large specimens (see page 58). 

 Fig 51 —Cells When such objects are in fluid, the removal 

 of the superfluous water is sufficient to make 

 the cover glass^adhere to the cell. 



If insects are to be examined dry, m va 



the cover glass may be made to 



adhere to the cells by placing a ^lo. 52.-Slip wxth Cell 



smear of grease or vaseline around "^ °^^^' 



the upper edge. , ■ i i 



For the examination of aquatic weeds, algae, and ammalcula Trough, 

 with low powers, a trough is a useful apparatus. Fig. 53 shows a 



