On the Estimation of Aperture. By the late C. Hochin, jun. 347 



that the one is measured in one dimension, the other "in two 

 dimensions," as is said by the author. The author makes the same 

 difference : (1) by excluding the intensity of the rays, and (2) by 

 introducing the square root of the photometrical equivalents of the 

 angles as the measure of " aperture." 



This being understood, I should agree that it is better to base 

 the definition of numerical aperture upon photometrical principles 

 directly, instead of on an indirect demonstration, by means of the 

 ratio of linear aperture to focal length, which ratio should be con- 

 sidered as a secondary expression of aperture. 



