4 APPARATUS 



dry or immersion lens with a low power (%-inch) 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 differ- 

 ence 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 ^j-inch 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 J^2~ mc h or a high- 

 power dry lens. 



It is well, however, in a bacterial or blood preparation to first examine the smear 

 with the %-inch objective in order to determine suitable areas for examination with 

 the oil-immersion 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. 



After using the oil-immersion objective the lens should be wiped clean of oil 

 with a strip of Japanese lens paper or with a silk handkerchief. If the oil should 

 dry on the surface of the lens it may be removed with a drop of xylol on a piece of 

 lens paper. Immediately afterward the lens should be dried. Dried oil on a lens 

 often causes the lens to be considered defective. Accidental contact of the dry 

 objectives with oil is not uncommon and should always be thought of when satis- 

 factory optical effects are not obtainable. In depositing the drop of immersion oil 

 on the slide bubbles are at times formed which make it almost impossible to use the 

 H 2-inch objective. Under such circumstances I either prick the bubbles or wipe off 

 the oil and deposit a drop anew. 



It is advisable to cultivate the use of both eyes in doing microscopical work. 

 When using one eye the other should be kept open with accommodation relaxed. 

 It is this squinting of the unemployed eye which so often fatigues. A strip of card- 



