UNEQUAL INTENSITY OF EMITTED LIGHT 



of light alone more light can always be obtained in the image 1 > y 

 throwing more upon the object but 110 increase in the amount of illu- 

 mination can make a dry lens equal in performance as regards aperture 

 to a wide-angled immersion lens. 



The popular notion of a pencil of light may be illustrated by 

 fig. 33, which assumes that there is equal intensity of emission in 

 all directions, and that the intensity of a portion of the pencil taken 

 close to the perpendicular is identical with that of another portion 

 of equal angular extension, but more removed from the perpendicular. 

 On this view, therefore, the (jinmtiti/ of Uyht contained in any given 

 pencils may be compared by simply comparing the contents of the 

 solid cones. 



When, however, aperture is considered, and the values of ,/ sin n 

 are worked out for particular cases, they are seen to differ from 



FIG. 33. Diagram showing erroneous inference as to the intensity of emitted rays. 



those obtained by estimating in the above manner the amount of 

 light in the solid cones, and some perplexity naturally arises from 

 the supposition that the measure of the aperture of the objective does 

 not correspond to that of the quantity of light which it admits. 



All this arises from the mistaken assumption that a luminous 

 pencil is properly represented by fig. 33. 



In the last century (1760) Bouguet 1 and Lambert 2 established 

 the important fact that 

 with any surface of inti- 

 f or in. radiation the inten- 

 sity of the emitted rays is 

 not the same in all direc- 

 tions. The power of emis 

 siou and the intensity of 

 the rays (i.e. the quantity 

 of light emanating from 

 a given surface-element 

 within a cone of given 

 narrow angle) varies in 



the proportion of the co- 



1 . \ n ,-,. FIG. 34. rhe intensity of emitted rays is not the 



sine of the angle ot obliqui- same in ail directions. " 



ty under which the ray is 



emitted; in other words, in the proportion of the rosins of the angle 



of deflection from the perpendicular to the luminous surface under 



1 Traite d'Optiqtte snr la Gradation tie la Lumii'-re, 1760. 

 - Photometria, 1760. 



K 2 



