MICROSCOPIC METHODS 107 



scopes will show that in general the higher the power, that 

 is, the greater the magnification, the smaller is the opening 

 through which the light must pass at the lower end of the 

 objective. It is evident that with the higher powers the 

 amount of light which may enter the lens is considerably 

 reduced. It has been found in practice that the interposi- 

 tion of a drop of oil between the lens and the object to be 

 examined make a far more satisfactory definition, the object 

 being seen much more clearly. Inasmuch as oil must al- 

 ways be used with the highest power lens, and should never 

 be used with other lenses, a brief explanation of the reasons 

 for its use will be given. 



The diagram in figure 49 should be consulted. Let C 

 represent the glass cover slip over the object to be examined, 

 and L the lens through which light must pass to the eye 

 of the observer. The diagram shows a drop of oil, H, lying 

 to the left of the line NB. This oil is usually oil of cedar 

 wood, having the same refractive index as the glass used 

 in the slide. By means of the mirror and the substage con- 

 denser the rays of light are focused upon the object which 

 is to be examined. These rays of light may be represented 

 by AB and A'B. Eays of light which are normal, that is, 

 perpendicular to the surface of C, such as the ray BN, will 

 pass straight through without refraction in glass, air, or 

 oil. A ray such as AB, however, upon entering the denser 

 medium, glass, will be refracted in the path BD. Here it 

 enters the air and is again refracted in a direction parallel 

 to the incident ray, that is, in the direction of DE. If, 

 however, a drop of oil replaces the air this second refrac- 

 tion would not occur. This is illustrated by following the 

 ray A'B which is refracted in the glass as BD'. Upon 

 entering the oil it continues in the same direction as the ray 

 BF. It is evident that all rays lying between A'B and BN 

 and N'N wiU enter the lens L when oil is present, but that 

 only a portion will enter if air is between the lens and the 



