474 THE STORY OF PLANT LIFE 



the objective. Proceed more cautiously in focussing the 

 object with a high power, and move the tube or eye-piece 

 as well as the slide, which must be centred by the stage 

 adjustment. Keep both eyes open when viewing the object, 

 and do not close the one not in use. 



The necessary processes involved in the preparation of a 

 microscopical slide — assuming this is undertaken, though 

 not necessary in examining fresh material, which can be 

 examined in water or other media in a cell or moist 

 chamber — includes preservation, fixing or hardening, cutting 

 of sections, and mounting. 



As a rule, fresh material is best, but if it has to be kept 

 it may be preserved in alcohol, when it becomes already 

 dehydrated, and at the same time the chlorophyll is extracted, 

 whilst the cell-contents may become disorganised. A better 

 medium is formalin (40 per cent, solution) used as a 4 per 

 cent, solution. 



When ready to be used the material should be rinsed with 

 water. If alcohol is used it should be 50 per cent, dilute. 



For fixing and hardening contents before preserving in 

 alcohol or formol, the following may be used : Picric acid, 

 saturated solution chromic acid (o-i to 0*5 per cent, solution 

 in water), osmic acid (o*i to i per cent, solution in water). 

 The object is kept in this for twenty-four hours in 300 c.c. 

 water with 2 grm. chromic acid and 3 c.c. glacial acetic acid, 

 then washed in running water, then placed/^r diem respect- 

 ively in 30 per cent., 50 per cent, and 70 per cent, alcohol, 

 then it may be finally preserved in 95 per cent, or absolute 

 alcohol. Objects so treated are more brittle than when 

 kept in formol and fixed as above. This defect may be 

 remedied by treating with glycerine, if the section is to be 

 mounted in glycerine (which should be dilute). 



The next operation is to cut a section, or rather three 

 sections — transverse, radial longitudinal, and tangential 



