28 On the Halo seen around all Bodies. 
ible that follow the edge or surface of the body on which the halo 
rests. Such is the nature of this extraordinary power, that no 
confusion arises from this crossing of the lines, for as I have fre- 
quently observed, only those are seen which are parallel to the 
edge whether the object on which they rest is in a vertical or 
horizontal position. 
33. It is not the external surface alone on which the halo rests; 
all the inner and outer circumferences of flat bodies, and of all 
round or irregularly shaped apertures, are edged and lined by this 
diaphanous, lenticular halo. There is no opening so small, nor 
any so large, that is not edged with it; but its peculiar character 
as a perfect lens, is to be found only when the aperture is ofa _ 
certain diameter. Its own diameter being limited, it becomes ne- 
cessary that the aperture shall be of such a size as to allow the 
halo to fill it up completely, which it could not do if the aperture 
were too large. 
34. The rays that proceed from this isla, when it lines an | 
aperture, converge to a focus nearer or farther from this aperture 
according to its size, and according to its distance from the eye, _ 
It is from this focus that an image of the sun—as it is called—is 
seen, which image is either thrown on the floor from the hole in _ 
the window shutter of a dark room, or on the ground under trees, 
The same spot of bright light is projected on the wall from the 
slats of a window blind, at sun-rise, or sun-set—that is, if the 
slats are near together, or if there be acrack in one of them. _ 
When the points of contact of these slats or cracks, are close to 
one another, the interstices between the points become lenses, and 
from these focal points small dense circles of light are seen. 
In cutting a slit i in a oud we press the knife on it unequally— 
making a feebler inc ‘every half inch—on looking through — 
the slit we shall perceive that it i is composed of circular halos, in- 
stead of a long line of misty light, as is always the case when the 
pressure of the knife i is equal, and the slit is of the same diameter 
— 
. It has frequently been asked, why the circular spots of light 
a trees have that perfect regularity of outline, when the open- 
ing between the leaves are generally angular. I have~ already 
said that they are not the representatives of the apertures, but of ‘ 
the condensation of light at each focal point. It is the divergent 
— this focus that throw the shadows of lenses, such as ae 4 
