60 PHOTOMICROGRAPHY 



estimate can be made of the magnification produced 

 in the photomicrograph of any object, when the plate 

 is placed at a given distance (say, 10", or 250 mm.) from 

 the shoulder of the objective, by dividing this distance 

 by the focal length of the objective. Thus, a J" objective 

 will give an amplification of 10-i-J=60 times, and a 

 2 mm. will give 250^-2 = 125 times under these con- 

 ditions when used without an eyepiece. 



Numerical Aperture. Most objectives are marked by 

 a figure preceded by N.A., and this figure is of the greatest 

 importance, as it indicates the properties of the lens 

 in a way unattainable even by long description. It 

 is found that the wider the cone formed by the extreme 

 rays proceeding from a point which the objective can 

 utilise, the better, and that the perfection of the image 

 depends on the amount of light so gathered. The number 

 of degrees in the angle of the cone was used at one time 

 as a measure of the " aperture " of an objective, but 

 it did not give any direct comparison of the properties 

 of dry and immersion objectives. Prof. Abbe con- 

 sequently substituted for it the sine of half the angle of 

 aperture multiplied by the refractive index of the medium 

 (air, oil, etc.) in which the objective works, and called 

 this value " numerical aperture," so that 



N.A. n sin a 



where a =half the angle of aperture, 

 and n=the refractive index of the immersion fluid. 



The following properties depend on the width of the 

 cone of light gathered by the objective, and of them the 

 " numerical aperture " forms a measure. 



Resolving Power. Varies directly as the N.A. 



Brightness of Image. Varies as the square of fche N.A. 

 for any given magnification. 



The following properties generally become less as 

 the N.A. increases : 



