USE OF THE MICROSCOPE 3 



ive 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. 



In using the oil-immersion objective always dip the lens in the oil and practically 

 touch the cover-glass the eye being at a level with the stage before beginning 

 to focus. With the coarse adjustment one can feel the contact with the cover- 

 glass, which is impossible with the fine adjustment. It saves time and disappoint- 

 ment to make a preliminary examination of a preparation requiring the high dry 

 or immersion lens with a low power (2/3-in.) before employing the higher power; 

 in this way we locate or center a suitable field for study. 



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 enter- 

 ing the objective, as would be the case if we used a dry objective with an interven- 

 ing air space. In this case a portion of the rays would be turned aside by the dif- 

 ference 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 i/6-in. of high 

 numerical aperture there may not be sufficient working distance 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. The skill of the optician, however, can obviate this 

 defect in an objective of high numerical aperture so that it may combine the qualities 

 of perfect definition with sufficient working distance. 



Practical Points in the Use of the Microscope. 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/i2-in. 

 or a high-power dry lens. It is well, however, in a bacterial or blood 

 preparation to first examine the smear with the 2/3-in. objective in 

 order to determine suitable areas for examination with the oil-immer- 

 sion objective. With tissue sections 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 glass, before using the microscope, will give a 

 surprising amount of information. 



