INTRODUCTION TO PHYSIOLOGICAL OPTICS 435 



NUMBERING OF PRISMS AND LENSES 



Numbering of Prisms. Prisms may be num- 

 bered according to refracting angles or according 

 to the extent to which they turn the light ray 

 from its course (angular deviation). Angular 

 deviation is expressed by the methods of Dennett 

 and of Prentice. 



Dennett's method. The length of an arc of 

 57.295 equals its radius of curvature. A prism 

 which will bend the ray one hundredth part of 

 this arc is called one centrad. The angular 

 deviation produced by the prisms are by this 

 method expressed in hundredths of the radius 

 measured on the arc. 



Prentice's method. The unit of comparison is 

 a prism-dioptre, i. e. a prism that deflects a ray 

 of light one centimetre at a plane one metre dis- 



O A 



tant, or, in other words, the hundredth part of 

 the radius measured on the tangent. 



Numbering of Lenses. Lenses are numbered 

 according to their refractive power. The unit is 

 a lens with a focal distance of one metre. This 

 unit is termed a dioptre, D. A lens of two 

 metres focus is one half the refractive power, or 

 \ D. The lenses ordinarily employed in ophthal- 

 mic practice extend from 0.12 D to 22 D. The 



