PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 773 



and less complete toward the extremes of the visible spectrum, and there are also 

 residual imperfections in the fusion of the rays. The apochroniatic objectives repre- 

 sent the highest order of correction, and the semiapochromats are about intermediate 

 in the quality of correction. 



Objectives are listed according to their optical characteristics such as primary 

 magnification, numerical aperture, focal lengths, and whether "dry" or "immer- 

 sion." The term dry signifies that the objective, when properly used, is separated 

 from the specimen by a stratum of air. In the case of immersion objectives some one 

 fluid for which medium the objective has been computed, such as water, glycerin, 

 cedarwood oil, etc., is used to connect the front lens of the objective with the specimen. 



Resolving Power. — The ability of an objective to resolve detail is dependent, theo- 

 retically, upon the numerical aperture of the objective and the wavelength of light 

 used. 



The relation is expressed numerically by the equation 



n = -^^ (1) 



and shows that, if the numerical aperture N.A. of the objective is increased or if the 

 wavelength X of the light is decreased, the number of lines 71 capable of being resolved 

 will be increased. 



This theoretical resolving ability might well be termed "potential resolving abil- 

 ity"; the ability to resolve is inherent in the lens but whether it is achieved in practice 

 is quite another matter. In theory two things are of interest: numerical aperture and 

 wavelength of light. 



The fundamental difference between a dry objective and an immersion objective 

 is one of resolving power. An immersion objective has greater light-gathering power 

 than a dry lens of corresponding focal length. This light-gathering power is expressed 

 by the numerical aperture. 



The present commercial limit for numerical aperture is 1.40 for the best apochro- 

 matic objectives. Objectives of 1.60 N.A. are available for metallurgical work. 



The apochromats of 1.40 N.A. have a primary magnification of about ninety times 

 so that they not only possess the ability to resolve but also the ability to magnify 

 greatly. The full magnifying power of the optical combination is secured almost 

 irrespective of all other conditions. The specimen may be well prepared or very 

 poorly prepared; the illumination may be critical, or it may be very poorly arranged; 

 the operator may be skillful or unskillful; and many other combinations may occur 

 short of total disruption which will not alter the magnification but which ,vill impair 

 very seriously, if not wholly ruin, definition and consequently the resolving ability of 

 the combination. 



Numerical Aperture. — Figure 2 shows the conditions preA^ailing in a metallurgical 

 microscope where light is directed by suitable nieans to the prepared surface of the 

 specimen from which it is reflected. Two rays, such as rays 1 and 2 leaving the object 

 at the same angle, will behave quite differentl3^ Ray 2 is refracted by the immersion 

 oil or bent inward. Ray 1, which is pictured to show the conditions without immer- 

 sion oil, just enters the front lens of the objective. Other rays leaving at a greater 

 angle than ray 2 will also be bent inward, and some extreme ray, such as ray 3, will just 

 enter the front lens of the objective. 



Figure 3 illustrates numerical aperture when the specimen is viewed by transmitted 

 light. In this case the specimen is mounted on a glass slide, which is placed on the 

 stage of the microscope and is illuminated through the substage condenser. For 

 purpose of illustration the right half of the front lens of the objective is assumed to be 

 that of a dry lens and the left half that of an immersion objective. Thus an immersion 



