18 PROPERTIES OF LENSES. 



The angular aperture is the capacity, which a lens 

 has for receiving rays from the object and transmitting 

 them to the image, and is determined by the ratio 

 between the diameter of the opening and the focal 

 length. 



The apparent depth of focus of an objective, is 

 in inverse ratio to its magnifying power, and as the 

 utility of the higher powers depends upon their capa- 

 bility for showing the minute surface-structure of 

 markings, and as this increases with the power 

 the objective has of collecting a large number of 

 rays from an object, in other words on its angular 

 aperture, we are obliged to sacrifice depth of 

 focus, in the attainment of magnifying power and 

 angular aperture. 



The immersion principle where fluid is placed be- 

 tween the object and objective increases the angular 

 aperture, as these objectives can be made with larger 

 fronts than dry ones of the same focal length. The 

 homogeneous immersion principle is to be preferred, 

 as it increases the angular aperture as much again as 

 water, and the oil has the great advantage of not 

 evaporating when prolonged investigations are ne- 

 cessary ; care should be taken that the smallest 

 quantity sufficient to cover the aperture of the ob- 

 jective, when focussed on the object, is used, as any 

 excess would cause blurs of irradiation, not to men- 

 tion other objections. 



The old method of designating the angular aper- 

 ture of an objective, by the number of degrees in 

 the angle formed by the most lateral rays meeting 



