INTRODUCTION TO PHYSIOLOGICAL OPTICS 417 



of each hypothetical prism is directed to the 

 periphery of the lens. As the periphery is ap- 

 proached the refracting angles increase, and 

 hence refraction increases. The increased refrac- 

 tion of the outer rays diverging from a luminous 

 point compensates in part for their greater angle 

 of incidence, and hence most of the rays converge 

 approximately to the same focus. 



Estimation of Principal Focal Distance. Re- 

 move from the lantern the tubes holding the projec- 

 tion lenses. Place in front of the condensing lens 

 the diaphragm with [-shaped aperture each limb 

 of which is 5 mm. long and 1 mm. broad. Place 

 the convex lens in the axis of the pencil emerg- 

 ing from the illuminated slit and at a distance 

 from it a little greater than the principal focal dis- 

 tance as determined roughly in the preceding ex- 

 periment. On the other side of the lens place a 

 screen at such a distance as to give a strongly 

 enlarged clear picture of the |_. Measure 



I = the length of one limb of the |__, 

 L = the length of its image, 

 A = the distance of the screen from the lens, 

 / = the principal focal distance of the lens, 



then * f= A T , T 



JL/ 4" tr 



1 This formula is derived as follows : Let a be the distance 

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