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 unless the object is thoroughly dried a cover glass 

 may be required to prevent the object from floating off 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 Examination 

 a cover glass and a slip, the superfluous fluid around the cover fluj^^^*^** ^ 

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

 attraction wiU 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, Slip 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 

 hollows. They are used in the 



Fig. 50.— No. 3405. 



Fig. 51.— Cells. 



same way as ordinary slips, the water which 

 fifls the cavity holding the cover glass in 

 position by capillary attraction. 



CeUs or rings of vulcanite metal or glass Ceiis. 

 may be cemented to 3 X 1-inch slips with 

 Hollis glue, forming deeper cavities for the 

 reception of large specimens (see page 58). 

 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, 



the cover glass may be made to 



adhere to the ceUs by placing a 



smear of grease or vaseline around 



the upper edge. 



For the examination of aquatic weeds, algse, and animalcula Trough. 



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



m 



a. 



Fig. 



52.— Slip with Cell 

 and Cover. 



