24 On the Halo seen around all Bodies. 



constant motion. That the conjunctival fluid is pushed together 

 so as to form a ridge, can be proved by looking at the clear sky, 

 or a candle, through a small pin-hole in a card. On closing the 

 lids slowly and then suddenly opening them, this ridge will be 

 seen on the enlarged diameter of the pin-hole. 



10. I have stated in section 4th, that the halo is always of the 

 same density and diameter at a certain point of view, and that on 

 opening the closed eye it will suddenly contract, and that the 

 light diverging from it, will be much brighter. This contraction 

 and expansion is not confined to those halos which adhere to 

 edges, or to a plane surface, for all narrow slits, all small circular 

 holes, contract and expand under similar circumstances. - 



11. Light falls on the halo precisely as it does on a bright 

 brass ball. It is well known that if we breathe on the ball and 

 pass the hand over it horizontally, the light from a self-luminous 

 body will fall on it vertically. If the hand is passed over it ver- 

 tically, the light will fall horizontally. This phenomenon does 

 not arise from the presence of moisture, for the same thing occurs 

 whether we breathe on the ball or not, although then not so per- 

 ceptible. There are always inequalities even on the smoothest sur- 

 face on which light would glance, but the peculiar feature of a 

 beam of light is more clearly defined when the hand is passed 

 over a moist surface ; the ridges are then formed more distinctly. 



12. The cause of the formation of the ridges must be very 

 obvious ; the hand in passing across the ball, cannot come in con- 

 tact with every part of the surface ; a number of elevations or 

 ridges therefore will arise parallel to the motion of the hand, and 

 it is across these ridges, at right angles, that light falls. 



13. But on account of the sphericity of the ball, the beam of 

 light will be of very narrow diameter, as it can only glance over 

 a very circumscribed area, which area, however, is the prominent 

 point of the ball. 



14. The halo accompanies every one of the ridges following 

 these elevations and depressions. It is always parallel to every 

 plane or curved surface and edge, and the lines intersect each 

 other at every angular point and around every curve or sphere 

 without interfering. 



15. If it be a plane surface on which we breathe, and the 

 hand is passed over it vertically, the light will glance across the 

 whole mass of ridges in a horizontal direction. A contrary ac- 



