ILLUMINATION BY TRANSMITTED LIGHT. 



107 



graphic works should still refer to parallel or divergent light, which 

 is to be applied, according to circumstances, for the illumination of 

 the fi eld of view (i.e., probably its surface-elements). The light which 

 illuminates a given point of the field of view is always convergent, 

 i.e., the rays proceeding from the mirror intersect in that point, 

 and diverge from it towards the objective. The converse supposi- 

 tion hardly merits confuting. 



Secondly, as regards the different lenses or systems of lenses 

 usually inserted in the path of the incident light, it may readily be 

 shown that, like the form and 

 position of the mirror, they 

 exercise influence only in cer- 

 tain cases, while on the other 

 hand they are inoperative in 

 all cases where the mirror and 

 the source of light may be L* 

 regarded as unlimited. If p 

 (Fig. 45) is a surface-element 

 of the field of view, and a b 

 the diaphragm determining the 

 aperture of the incident cone 

 of light, then the first refract- 

 ing or reflecting surface below 

 the diaphragm acts as an un- 

 limited source of light. For 

 every point of it receives rays p IG 



from all directions which come 

 into account, and consequently emits rays in all corresponding 

 directions. It is quite immaterial whether the pencils reach the 

 surface directly or after various refractions or reflexions, for the 

 difference of direction in the incident rays does not come into 

 consideration, the real source of light being unlimited. We can, 

 moreover, regard any plane below the diaphragm (the diaphragm 

 itself included) as an illuminating surface, whether a deviation 

 caused by refraction or reflexion takes place in it, or not. If, 

 therefore, we place our illuminating surface temporarily in the 

 plane of the diaphragm, every point of the latter acts as a self- 

 luminous point, and the illuminating lenses L L and R R can 

 clearly have no other action than that of causing the rays, which 

 serve to illuminate the point p, to undergo several deviations on 



