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 iiuier 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 of a 

 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 halo, when it lines an 

 aperture, converge to a focus nearer or farther from this aperture 

 accordmg 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 a crack 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 the small dense circles of light are seen. 

 In cutting a slit in a card, if we press the knife on it unequally — 

 making a feebler incision every half inch — on looking through 

 the slit we shall perceive that it 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 is equal, and the slit is of the same diameter 

 throughout. 



35. It has frequently been asked, why the circular spots of light 

 under 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 

 rays from this focus that throw the shadows of lenses, such as are 



