the Inflection, Reflection, and Colours of Light. 253 
proached to it, either between the sun and it, or the shadow 
and it, or on the same line with it, the shadow of the one 
body comes out a considerable way, and meets that of the 
other. Now it is evident, that when the bodies are held at a 
sufficient distance from one another, a penumbra is formed 
round the shadow of each, making it less than it should be 
were there no inflection ; but when the bodies are brought so 
close to one another that the edge of the one is within the 
sphere of the other's inflection, the light being already bent 
by this last, the former can have none to bend, and con- 
sequently no penumbra in the part of the shadow correspond- 
ing to that part of the body which is within the other's sphere 
of inflection ; and the rest of the shadow having a penumbra, 
this part that has none will be larger than it, and increase as 
the bodies approach, till at last it meets the other shadow; 
the like appearance happening when the shadows are thrown 
on the eye. Mr. Melvill has endeavoured to show that it 
belongs simply to a case of* vision;* however, we have now 
seen that it has no reference to the structure or position of 
the eye, but only to the common nature of all shadows. 
Ohs. 11. If we shut out all the light coming into a room 
from external objects, except what may pass through a small 
hole of f or ^th of an inch in diameter, the images of the ex- 
ternal objects, as clouds, houses, trees, will be painted on the 
opposite wall, by the rays of light crossing at the hole ; but if 
a piece of rough glass, or of very fine paper, be held so as to 
cover it all over, the light does not pass through ; then if the 
paper be wetted with oil, or the glass with water, so as to give 
either a small degree of transparency, the first rays that come 
• Edinburgh Literary Essays, Vol. II. 
