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tensity of light ; because in calculating this function, we have abstracted from all physical 

 causes not included in that fundamental law of catoptrics, which is expressed by our original 

 equation, 



cos. ^l + COS. ^'1 = 2 COS. I. COS. nl, (A), [1] : 



or in the resulting formula 



tcdx + /3dj/ + ydz ■=! dV, 



«, ^, y being the cosines of the angles which the ray passing through the point xyz makes with 

 the axes of coordinates, and V the characteristic function. To distinguish therefore that mathe- 

 matical affection of the system to which the preceding calculations relate, from the physical 

 intensity of which it is an element, we may give to it a separate name, and call it the Geome- 

 trical Density. 



