IN OPTICS. 117 



case intercepted, the effect of the latter rays, in 

 giving colour to the middle of the picture, will 

 consequently be rendered more evident. Hence 

 it is, that, when a luminous point is not suf- 

 ficiently remote for distinct vision, the seeming 

 shadow upon it, occasioned by any small opake 

 object held before my eye, is always blue; and 

 this is the second of the appearances, which I 

 said are frequently to be observed, in experi- 

 ments upon luminous points. 



