Till. MICROSCOPE. 3 



the breaking <>f the cover-glass of a hanging-drop preparation and 

 consequent risk of infection is the attempt to focus with the fine ad- 

 justment. It should be an invariable rule for the worker to bring his 

 objective practically into contact with the upper surface of the cover- 

 glass, then using the coarse adjustment (rack and pinion) to slowly 

 elevate the objective, looking through the eye-piece at the same time. 

 In other words, obtain focus with the coarse adjustment and maintain 

 it with the fine adjustment (micrometer screw). The fine adjustment 

 should only be used after the focus is obtained. 



It will be observed that objectives frequently have their numerical 

 aperture marked on them. This is expressed by the letters N.A. 

 From a practical stand-point this gives the relative proportion of the 

 rays which proceeding from an object can enter the lens of the objective 

 and form the image. Of course, the greater the number of rays, the 

 greater the N.A., the better the definition, and consequently the better 

 the objective. Immersion oil, having the same index of refraction 

 (1.52) as glass, would not deflect rays coming from the object and so 

 prevent their entering the objective, as would be the case if we used a 

 dry objective with an intervening air .space. In this case a portion of 

 the rays would be turned aside by the difference in the refractive index 

 of air. As a rule, the higher the numerical aperture, the better the 

 objective and the less the working distance. In blood counting, the 

 cover-glass being comparatively thick, it may happen that with a 1/6 in. 

 of high numerical aperture there may not be sufficient working dis- 

 tance to bring the blood-cells into focus, which could be done with an 

 objective of lower numerical aperture. Consequently, we must 

 always consider the matter of working distance as well as that of 

 numerical aperture. 



An important matter in the use of the microscope is to get all the 

 details possible with a low power before using a higher power. This, 

 of course, does not apply to a bacterial preparation where it is necessary 

 to use a i/ 1 2-in. or a high-power dry lens. With tissue sections, how- 

 ever, it is not only advisable to begin the study with the lowest power, 

 but even an examination with the unaided eye or with a magnifying 

 before using the microscope, will give a surprising amount of 

 information. 



