2 THE MICROSCOPE. [January, 



the ocular. Cloudiness or particles of dust on any part of the 

 ocular may be detected in this way. 



Dust or Cloudiness on the Objective. — Employ the same 

 ocular and objective as before, and the fly's wing as object. 

 Focus and light well, and observe carefully the appearance. Rub 

 glycerin on one side of a slide near the end. Hold the clean 

 side of this end close against the objective. The image will be 

 obscured, and cannot be made clear by focusing. Then use a 

 clean slide, and the image may be made clear by elevating the 

 body slightly. The obscurity produced in this way is like that 

 caused by clouding the front-lens of the objective. Dust would 

 make a dark patch on the image that would remain stationary 

 while the object or ocular was moved. 



If too small a diaphragm is employed, only the central part of 

 the field will be illuminated, and around the small light circle 

 will be seen a dark ring. 



Relative Position of Objects or parts of the same 

 object. — The general rule is that objects highest up come into 

 focus last in focusing up, first in focusing down. 



Objects Having Plane or Irregular Outlines. — As object 

 use three printed letters mounted in stairs in Canada balsam. 

 The first letter is placed directly upon the slide, and covered 

 with a small piece of glass about as thick as a slide. The second 

 letter is placed upon this and covered in like manner. The 

 third letter is placed upon the second thick cover and covered 

 with an ordinary cover-glass. The letters should be as near 

 together as possible, but not overlapping. Employ the same 

 ocular and objective as before. 



Lower the tube till the objective almost touches the top letter, 

 then look into the microscope, and slowly focus up. The lowest 

 letter will first appear, and then, as it disappears, the middle one 

 will appear, and so on. Focus down, and the top letter will 

 first appear, then the middle one, etc. The relative position of 

 objects is determined exactly in this way in practical work. 



Transparent Objects Having Curved Outlines. — The 

 success of these experiments will depend entirely upon the care 

 and skill used in preparing the objects, in lighting, and in focus- 



Employ a 5 mm. (1-5 in.) or higher objective and a high 

 ocular for all the experiments. It may be necessary to shade the 

 object to get satisfactory results. When a diaphragm is used the 

 opening should be small. 



Air Bubbles. — Prepare these by placing a drop of thin 

 mucilage on the centre of a slide and beating it with a scalpel 

 blade until the mucilage looks milky from the inclusion of air 

 bubbles. Put on a cover-glass but do not press it down. 



