APPENDIX 



553 



the tube, and an ocular or eye-piece which slips into the upper end. 

 Objectives and oculars are of various powers, and an objective of high 

 power may be used with an eye-piece of low power, or vice versa. 

 The lowest magnification obtainable in 

 a student's microscope is usually about 

 50-80 diameters, the highest about 

 250-300. The object is placed upon 

 the stage and brought into focus by 

 raising and lowering the tube. Coarse 

 adjustment is effected by sliding the 

 tube, either directly or by a rack and 

 pinion ; fine adjustment by raising or 

 lowering the arm which holds the tube. 

 This is done by a screw which works 

 against a concealed spring. With the 

 high power the objective is closer to 

 the object, when it is in focus, than 

 with the low. An object may be viewed 

 either by reflected light falling upon it 

 from above, or, if it be transparent, by 

 transmitted light cast through it from 

 below by the mirror. The object is 

 placed or mounted upon a glass slide. 

 Usually it is immersed in some medium 

 which is either temporarily or perman- 

 ently fluid (see p. 554). In this case it 

 must be protected by a coverslip of thin 

 glass. 



In many cases it is desirable to stain 



the object. A few re- 

 Staining, agents, such as methylene 



blue, will stain living ob- 

 jects ; for most it is necessary that the 

 animal or tissue should be killed. This 

 is done with a fixing agent, a strong 

 poison that kills rapidly and so allows 

 only the minimum of change to take 

 place in the object. Saturated solution 

 of corrosive sublimate in water, 2 per 

 cent, solution of osmic acid in water, 

 I per cent, solution of glacial acetic 

 acid in water, absolute alcohol, and other 

 substances, are used for this purpose. 

 Osmic acid is useful for small animals ; 

 for tissues, a mixture of nine parts cor- 

 rosive sublimate solution and one part glacial acetic acid is a good 

 reagent. The specimen must be thoroughly washed to rid it of 

 all traces of the fixative before staining. Carmine, and logwood or 

 hsematoxylin are common stains. Various preparations of each of these 

 are in use for different purposes ; they can be bought ready made, and 

 directions for preparing and using them may be found in books, such as 



Fig. 398B. — Side view of a 

 compound microscope, 

 with detached eye-piece 

 and objective. 



«., Milled rim of screw of fine ad- 

 justment ; h„ base of stand ; 

 c, collar ; d.. diaphragm which 

 can be turned to bring between 

 the mirror and the object holes 

 of various sizes, one of which is 

 shown ; e., eye-piece or ocular, 

 above the end of the tube into 

 which it slides ; wz., mirror ; o. , 

 position of opening in stage ; 

 ob., objective, below the end of 

 the tube into which it screws ; 

 p., portion of pillar fixed to 

 base ; p' ., movable portion, 

 raised or lowered by fine ad- 

 justment ; j., stage ; r., tube. 



