On the Halo seen around all Bodies. 27 



28. This elongated lens does not, like a round one, give an in- 

 verted image of the pin ; it only reverses the movements. If a slit 

 is cut in a card with a sharp knife, and we look through it at a 

 candle or at the clear sky, it will be perceived that the halo is 

 present there also, and that its lines are parallel with the slit. On 

 moving a pin between the eye and the slit, the shadow of the pin 

 will be seen on it, but its movement will be the reverse of the 

 true one. In this experiment, as in the preceding one, the shadow 

 of the body of the pin is erect, and there is no representation of 

 the head. 



29. This contrary motion of object and shadow — this lenticu- 

 larity — does not belong exclusively to the overlapping of halos 

 and to narrow slits, for I have ascertained beyond a doubt, that 

 this nebula or halo, which exists on the edges of all bodies, pos- 

 sesses this singular power likewise. Even on the edges of our 

 fingers, and between the fingers, as may be ascertained by holding 

 up our hand before a candle, and moving a pin between the eye 

 and the finger, the halo there seen is an elongated lens, possessing 

 one lenticular movement. 



30. But there is a great difierence between the halos attached 

 to the edges and surfaces of bodies, and those belonging to small 

 circular apertures. The circularity, by making the rays of light 

 from all the foci of the interstices of the halo converge to one focus, 

 and thence radiate, reverses both the image and the movements. 

 This circular opening — a pin-hole for instance — is then a true 

 lens, with anterior and posterior convex surfaces, the convexity 

 of which gives the halo its lenticular or magnifying power. 



31. In whatever manner the lines of this lenticular halo are 

 produced — whether owing to refraction, reflexion, or whether it 

 be multiplication of outline — the fact is certain that they do exist, 

 and that the halo in which they are seen is a true lens. Turn a 

 glass globe which way- we will, this halo is always parallel to 

 the edge of the surface, and yet the globe has no edge, nor in 

 fact, is there an edge to any round body, such as a pin or a pen- 

 cil, sharp as it appears when close to the eye. 



32. The halo is of a certain depth, and always retains its diam- 

 eter and character, for the lines are forever parallel and the inter- 

 stices lenticular. Although we are certain from every variety of 

 careful experiment, that the lines cross each other in every direc- 

 tion, yet after the manner of rays of light, only those lines are vis- 



