20 



SOIOPTICON MANUAL. 



UNEQUAL ILLUMINATION. 



We can see that the brightness of an image decreases 

 from the centre towards the margin. The diameter of 

 the pencil g g passing through the lens parallel to its 

 axis, is of the same size as the opening of the stop , 

 and exceeds the diameter of the oblique bundle of rays. 

 Besides, the oblique rays lose considerable light by re- 

 flection, which may in part be re-reflected upon the image, 



detracting from its distinctness. With a short exposure 

 in the camera, this unequal illumination causes an under- 

 exposure at the margin. In the Sciopticon it is even 

 exaggerated by the reflector, but we generally wish the 

 objects occupying the central portion of the "field of 

 view" to stand out more clearly in the illuminated disk. 



CORRECTIONS 1JS THE EYE. 



Spherical aberration and distortion in the eye are 

 corrected (it Is believed) by the greater density of the 

 crystalline lens about its centre than towards the margin, 

 where the refracting angle would otherwise be too great. 



Chromatic aberration is corrected by the combined 



