1890.] MICROSCOPICAL JOUENAL. 223 



amongst the histological elements. Such a course as this tends to a 

 gradually increasing interest, while the reverse generally ends in a set- 

 tled indifference, because the evolution of a natural interest has been 

 stifled by a premature attempt to mentally grasp what only a trained 

 eye can comprehend. 



Before the student commences the work of his microscopical obser- 

 vations, it is necessary that he should provide himself with a few in- 

 dispensable appliances ; to assist him a brief enumeration may save 

 trouble : 



Glass Slides. — Before observing an}^ transparent object under the 

 microscope, it must be supported on the stage of the instrument on a 

 glass slide beneath the objective. These glass slips, procurable of any 

 optician, are of a standard size of three inches long by one inch wide. 

 They should be of patent plate, free from bubbles and striae, and ground 

 at the edges. They are not expensive, as they may be cleaned and 

 used over again should the mounted object not be satisfoctory. If they 

 have been used for Canada balsam mounting, a short immersion in 

 methylated spirit, to which some liquor potass£e has been added, will 

 quickly remove the mounting medium, when the slide may be washed 

 in warm water and wiped clean, ready for future use. 



If the object under examination be opaque, such as seeds, pollen, 

 grains of flowers, eggs of insects, and the like, the rays of light may 

 be concentrated on them by the condensing lens, which is suflScient 

 preparation: indeed, a juster idea of the nature of the object may be 

 formed by this method than by any other ; but if the object is intended 

 to be examined by transmitted light reflected from the mirror beneath 

 the stage, then it is desirable to place the object under a covering glass 

 to keep it flat. 



Cover- Glasses. — These also, obtainable at the optician's, are of difler- 

 ent degrees of thickness, var3'ing from one-fiftieth to one three-hundredth 

 of an inch in thickness. Unless the magnifying power employed be 

 greater than one-sixth of an inch focal length, it is advisable for the 

 novice to use the thickest kinds, as the thinner glass for use with the 

 higher powers of the microscope breaks so readily in cleaning, and it 

 is very necessary that cover-glasses should be absolutely clean. Cover- 

 glasses are square or circular, as the exigencies of the object may demand, 

 or the fancy of the student may desire. Some use square covers for 

 everything, but the circular covers have so many advantages that they 

 are to be prefened. These will be referred to at a later period, when 

 treating of mounting in cells and in fluid. When these cover-glasses 

 come from the optician's they are not clean, but before using they must 

 be scrupulously cleaned, by steeping them in warm spirits and water, 

 and with very delicate handling wiped dry with an old, soft silk hand- 

 kerchief, and stored away in a clean box till they are required for use. 

 A simple, but very safe, apparatus for cleaning the very thin glass with- 

 out breakage is formed by having two blocks of boxwood, having their 

 ends turned perfectly flat, and covered with buft' leather, upon which 

 has been rubbed a little putty powder. These surfaces being smooth 

 and perfectly flat, the thin glass may be rubbed between without the 

 chance of being fractured, when a slight dusting with the old handker- 

 chief suffices to remove any stray particles of dust. 



[_To be continued.^ 



