26 On the Halo seen around all Bodies. 
23. It acts upon the surface of another halo—more perceptible 
when they are near together—in consequence of a singular and | 
wonderful property which it possesses. Jt is lenticular in the 
direction of its parallelism with the edge of the body on which it 
rests! If weturna piece of glass, steel, or pasteboard, edgewise, 
we shall find that the lines of the halo are parallel to this edge, as 
edge it may be called, for it presents no more extent of surface 
than if it were an edge of only a line in diameter. If we turn — 
the edge so as to let it lie horizontally, still the line will be par- _ 
allel tothe edge, and the interstices will preserve their lenticular i 
property—the halo, therefore, on every edge is always lentiodlan§ “4 
in the direction of its lines. | 
24. If we hold a card in each hand, one near the eye and tial D 
other at a little distance from it, and then move them in a ho- — 
rizontal direction, so near as to shut out the light between them, 
the halo on the one card will appear to- swell out to meet the other. — 
25. Ona superficial view of this phenomenon we might be led — 
to adopt the explanation given of it by Mr. Melville. Inthe En- — 
cyclopedia Perthensis of 1816, page 412, fig. 9, plate 257, he has — 
given his theory, with a diagram. When we become convinced ae 
of the fact, that the interstices between the lines of every halo are 
constituted like lenses—being lenticular according to the parallel- 
ism of the lines with the edge or surface of the body on which it~ 
rests—we shall no longer be at a loss for the true theory of the 
swelling out of the edges of bodies to meet one another. — 
26. What I mean by lenticularity according to the parallelism 
of the halo with the edges of bodies, is this:—If the shadow of 
an object near the eye, is thrown on an object a few inches dis- 
tant from it, and we move either of them to the right or left, the 
shadow of the one nearest to the eye will rest on the other and: 
move ina direction contrary to the one we give it. 
27. Therefore, if a pin is held near the eye, and another is at @ 
little distance from it, the shadow of the former will be plain 
seen ou the halo of the matte moving contrarily to the motion y 
of this parallelism the shadow, though it has one nition’ m 
ment—whieh j is a reverse movement—is seen in an erect posit 
But I must observe here that this applies only to the body, oF 
shaft of the pin, as the head, for reasons hereafter to be explai 
i not = its shadow on pecs lens of a halo. 
Ea a 
