i 5 MICROSCOPE AND ACCESSORIES. 



LIGHTING AND FOCUSING. 

 FOCUSING. 



\ 37. Focusing is mutually arranging an object and the microscope so that a 

 clear image may be seen. 



With a simple microscope (? 4) either the object or the microscope or both may 

 be moved in order to see the image clearly, but with the compound microscope 

 the object more conveniently remains stationary on the stage, and the tube or 

 body of the microscope is raised or lowered (PI. II, Fig. 10). 



In general, the higher the power of the whole microscope whether simple or 

 compound, the nearer together must the object and objective be brought. With 

 the compound microscope, the higher the objective, and the longer the body of the 

 microscope, the nearer together must the object and the objective be brought. If 

 the oculars are not par-focal (§ 48), the higher the magnification of the ocular, the 

 nearer must object and objective be brought. 



\ 38. Working Distance. — By this is meant the space between the simple mi- 

 croscope and the object, or between the front-lens of the compound microscope 

 and the object, when the microscope is in focus. This working distance is always 

 considerably less than the equivalent focal length of the objective. For example, 

 the front-lens of a ^th in., or 6 mm. objective would not be ^(th inch, or 6 milli- 

 meters from the object when the microscope is in focus, but considerably less than 

 that distance. If there were no other reason than the limited working distance of 

 high objectives, it would be necessary to use very thin cover glasses over the ob- 

 ject. (See \ 16, 17). If too thick covers are used, it may be impossible to get an 

 objective near enough an object to get it in focus. For objects that admit of ex- 

 amination with high powers it is always better to use thin covers. 



RIGHTING. 



\ 39. Unmodified sunlight should not be employed except in special cases. 

 North light is best and most uuiform. When the sky is covered with white 

 clouds the light is most favorable. The light should come from the left ; but if 

 it is necessary to face the window a vertical, adjustable screen between the face 

 and the window is desirable. If artificial illumination must be employed, use a 

 lamp that gives a brilliant and steady light {\ 57). 



It is of the greatest importance and advantage for one who is to use the micro- 

 scope for serious work that he should comprehend and appreciate thoroughly the 

 various methods of illumination, and the special .appearances due to different 

 kinds of illumination. 



Depending on whether the light illuminating an object traverses the object or 

 is reflected upon it, and also whether the object is symetrically lighted, or lighted 

 more 011 one side than the other, light used in microscopy is designated as re- 

 flected and transmitted, axial and oblique. 



\ 40. Reflected, Incident or Direct Light.— By this is meant light reflected upon 

 the object in some way and then irregularly reflected from the object to the mi- 

 croscope. By this kind of light objects are ordinarily seen by the unaided eye, 

 and the objects are mostly opaque. In Vertebrate Histology (Part II), reflected 

 light is but little used ; but in the study of opaque objects, like whole insects, 

 etc., it is used a great deal. For low powers, ordinary daylight that naturally 

 falls upon the object, or is reflected or condensed upon it with a mirror or con- 



