400 THE COMPOUND MICROSCOPE 



The objectives most commonly in use are f in. and |- in., 1 

 which designations imply that, when focussed upon the object, 

 they are approximately two-thirds and one-sixth inch respec- 

 tively from the latter. These distances are the focal lengths, and 

 the smaller they are the greater is the magnifying capacity of the 

 lens. As a consequence, the longer; focus lens is often spoken of 

 as the low power, and the short focus lens as the high power. The 

 image formed by the objective is projected on to the eyepiece, 

 where it becomes further magnified. The amount of magnifi- 

 cation of the eyepiece is commonly indicated by a number 

 engraved upon it. 



The object to be examined, mounted on a glass-slip in water 

 or some other appropriate fluid and covered with a cover-glass, 

 is placed on the stage, and light is projected on to it from below 

 by means of the mirror. If a condenser is present, the flat side 

 of the mirror is employed, but if not the concave side. To 

 focus the object, gradually lower the tube by means of the coarse 

 adjustment till the image becomes clear, and then turn the milled 

 head to and fro until the image appears most distinct. Proceed 

 in the same way when using the high power, but exercise the 

 greatest care not to bring the objective in contact with the 

 cover-glass, and immediately the image begins to appear use the 

 fine adjustment only. The aperture of the diaphragm should 

 be diminished till the maximum amount of detail is visible, 

 whilst by adjusting the condenser the light reflected from the 

 mirror can be accurately focussed upon the object. 



In working with the microscope, it is best to accustom oneself 

 to employ either eye. When drawing, view the object with the 

 left, and sketch with the aid of the right, eye. As a first exercise 

 in microscopic observation, it is well to examine the small 

 air-bubbles almost invariably present in large numbers in a 

 drop of water. Under jthe low power these appear as black 

 dots or bright patches with broad dark margins ; this dark border 

 is due to refraction. Adjust the slide so that one of the smaller 

 bubbles is in the centre of the field of view, and turn the nose- 

 piece so as to view the bubble with the high-power lens. Using 

 the fine adjustment, it will be noted that at a high focus the 



1 For the study of Bacteria and other minute organisms, higher powers 

 are required, such as fs in. objectives. 



