The Microscope. 243 



tangled in the object itself. The beginner may mistake them 

 for something more important unless he makes himself familiar 

 with them at the beginning of his studies. 



The difference in the appearance of convex and concave bodies 

 is also important and useful. Microscopical and ordinary vision 

 differ so widely from each other that it is often impossible to de- 

 cide whether a surface is convex or concave, especially if the 

 object be uniformly covered with markings that may be bosses 

 or depressions ; sometimes the same trouble is experienced by 

 the naked eye. Grooves belong in the same category with de- 

 pressions. To whom belongs the credit for discovering the 

 means of discriminating between these conditions, I do not 

 know. My impression is that it is due to Prof John Quekett, 

 an English microscopist who is remembered for his microscopi- 

 cal work and discoveries. The means of discrimination however 

 ■are simple. When the objective is racked upward, a convex sur- 

 face will appear lighter ; a concavity will appear lighter when the 

 objective is focussed downward. 



It is often difficult to find a small object and bring it into the 

 iield. This is especially true with high powers, the trouble in- 

 creasing with the increase of magnification, because the actual 

 field of high power objectives is so small that the chances of 

 ■escape for the small object are much greater than are the micros- 

 copist's for capturing it, especially if the mechanical stage be not 

 used. The only recourse is to remove the high power objective^ 

 substitute a lower power, find the object, place it in the centre of 

 the field, and then to re-attach the high power lens, wh«n the 

 object sought should be somewhere within the illuminated space. 



The majority of objects are examined in water, glycerine, Can- 

 ada balsam or some other liquid medium. As a rule opaque 

 specimens, if they cannot be made transparent, are studied dry, 

 that is surrounded by air alone. Few naturally transparent ob- 

 jects are ever mounted dry, although there are exceptions to this 

 rule. When the specimen has been placed on the glass slip, and 

 immibed in the drop of Canada balsam or other preservative 

 medium, a thin glass cover is added to flatten the rounded sur- 

 face of the drop, to prevent the intrusion of dust and of moist- 

 ure, and to protect the front of the objective. The mounting 

 medium also has an important effect in an optical way. When 

 the preparation is dry, the object is placed on a slip within a 



