4 STUDENTS HISTOLOGY 



in place, about twelve inches high. If the height of the table 

 upon which it is placed and the chair of the observer be in a 

 proper relation, no discomfort need be experienced in using the 

 microscope in the vertical position. The form of condenser 

 invented by Abbe may be placed directly below the stage (Fig. 1, K) . 

 It consists of two or three lenses combined so as to focus the rays 

 coming from the plane mirror upon the object. The condenser 

 gives a very intense illumination over a small field, and is adapted 

 to bacteriological work, where a high -power oil -immersion objective 

 is required. An oil -immersion objective is a specially constructed 

 system of lenses, with which a layer of thickened oil of cedar wood 

 is placed between the lower surface of the objective and the upper 

 surface of the glass covering the object under examination. The 

 oil -immersion lens in general use has an equivalent focal length 

 of one -twelfth of an inch, and is usually designated as the iV-inch 

 oil -immersion. For similar reasons, the low -power is a f- or 

 f-inch, and the ordinary high-power is a i-, i-, or i-inch objective, 

 as the case may be. The condenser is not necessary, except with 

 the high-power oil-immersion objective. If used with the other 

 objectives, the illumination must be regulated by lowering the 

 condenser, closing the diaphragm, and substituting the concave for 

 the plane mirror, till a clear and satisfactory picture is secured. 



ADJUSTMENT OF THE MICROSCOPE 



The microscope should be placed in front of the observer, on a 

 table of such height that, when seated, he may, by slightly inclin- 

 ing the head, and without bending the body, bring the eye easily 

 over the eye -piece. The slightest straining of the body or neck 

 should be avoided. The light should always be taken from the 

 side, and it matters little which side. Clouds or clear sky serve as 

 the best source of light for our present work. Always avoid direct 

 sunlight. If artificial illumination be employed though it is not 

 advised for prolonged investigation a small coal -oil flame may be 

 tempered by blue glass, or better a Welsbach gas-burner with a 

 blue glass globe, or an incandescent electric light. 



ADJUSTMENT FOR ILLUMINATION 



It will be observed that there are two mirrors in the circular 

 frame below the stage one plane and the other concave, The 



