Illumination with Plane Mirror. By Dr. H. E. Frip-p. 749 



fore be inferred from the parallelism of the solar rays that cloud light 

 falls with parallel incidence, as may be affirmed in the case of the 

 window-pane which reflects direct sunlight. Nor can the distance 

 of the cloud be accepted as a sufficient cause of parallel incidence, 

 considering its many degrees of expanse, and its actual nearness to 

 the earth. On the contrary, it is self-evident that the different 

 intensity of light reflected from a cloud at, say, half a mile or three 

 miles distance, and its different angular magnitude at those dis- 

 tances, are infinitely more important elements in the calculation of 

 illuminating effect than the hypothesis of parallel incidence. lu 

 fact, the inconstant distance of the cloud is in itself a practical 

 refutation of the idea that such a cloud surface has a constant 

 illuminating power or conditions an invariably parallel incidence 

 of reflected rays. Is it possible to believe, for instance, that 

 from the widely spread extremities of a sunlit cloud subtending 

 perhaps 20^ of sky arc, and distant perhaps less than a mile, none 

 but parallel rays shall fall on the mirror ? Or — taking the meaning 

 of a parallel beam of light to be that its dimensions are the same 

 throughout its course — is it possible to accept the notion suggested 

 by sundry diagrams in our handbooks that two inches of sky or 

 cloud light are all that natiu'e offers for the illumination of micro- 

 scopic objects, and all that the plane or concave mirror is capable of 

 reflecting ? 



