OBJECTIVES AND OCULARS 5 



croscope is, however, controlled by the ocular, as will be seen 

 below. 



It would appear at first sight that the light-grasping power 

 of an objective is indicated by its angular aperture. Such is not 

 the case, for Abbe has proved that in comparing objectives as to 

 their light-grasping and transmitting power it is the sine of half 

 the angle of aperture which should be taken into account and not 

 the angular aperture; and further, that since objectives are 

 not all dry, the index of refraction of the medium between the 

 objective and the object must necessarily be considered. It is 

 therefore now conceded that the light-grasping and transmitting 

 power of an objective is equal to the refractive index of the 

 medium in which the objective dips multiplied by the sine of 

 half the angle of aperture. The product is what is known as 

 the Numerical Aperture and is expressed N.A. = n sin /*. 



If the above formula is accepted as true it is evident that 

 if the value of n is increased the numerical aperture will likewise 

 be increased. 



The light rays illuminating an object by transmission through 

 the preparation evidently pass from a denser medium (object) 

 to a rarer medium (air), and following the law of refraction are 

 bent away from the perpendicular. Hence part of these light 

 rays are lost, since they are bent so far that they cannot enter 

 the small front lens of the objective. To prevent this loss and 

 secure a brilliant image it is necessary, according to the formula 

 N.A. = n sin /z, to increase the value of n. Therefore, to 

 obtain very high powers, the substitution of some liquid for air 

 (n = i) between the objective and the preparation becomes 

 imperative in order that the image may be bright and distinct. 1 



Objectives permitting the use of a liquid in this manner 

 are known as immersion objectives. When water is employed 

 (n = 1.33) they are called water immersion, and when an oily 

 liquid, oil immersion. Usually the oil consists of slightly thick- 

 ened oil of cedar wood (n = 1.52), and since the refractive index 

 of glass object slides and cover glasses is approximately 1.52 



1 Abbe found that the brightness of the image varies as the square of the numer- 

 ical aperture. 



