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. It is lenticular in the 

 direction of its parallelisfii with the edge of the body on which it 

 rests ! If we turn a 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 to the edge, and the interstices will preserve their lenticular 

 property — the halo, therefore, on every edge is always lenticular 

 in the direction of its lines. 



24. If we hold a card in each hand, one near the eye and the 

 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. On a superficial view of this phenomenon we might be led 

 to adopt the explanation given of it by Mr. Melville. In the En- 

 cyclopedia Perthensis of 1816, page 412, fig. 9, plate 257, he has 

 given his theory, with a diagram. When we become convinced 

 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 in a direction contrary to the one we give it. 



27. Therefore, if a pin is held near the eye, and another is at a 

 little distance from it, the shadow of the former will be plainly 

 seen on the halo of the latter, moving contrarily to the motion we 

 give them. Here it will be observed that the lines in both halos, 

 and of course the pins themselves, are parallel, and in consequence 

 of this parallelism the shadow, though it has one lenticular move- 

 ment — which is a reverse movement — is seen in an erect position. 

 But I must observe here that this applies only to the body, or 

 shaft of the pin, as the head, for reasons hereafter to be explained, 

 does not cast its shadow on the oblong lens of a halo. 



