178 Mr. Young's Observations on Vision . 
becomes rigid by age, and this is sufficient to account for 
presbyopia, without any diminution of the humours ; although 
I do not deny the existence of this diminution, as a concomi- 
tant circumstance. 
I shall here beg leave to attempt the solution of some 
optical queries, which have not been much considered by 
authors. 
1. Musschenbroek asks. What is the cause of the lateral 
radiations which seem to adhere to a candle viewed with 
winking eyes ? I answer, the most conspicuous radiations are 
those which, diverging from below, form, each with a vertical 
line, an angle of about seven degrees ; this angle is equal to 
that which the edges of the eyelids when closed make with a 
horizontal line ; and the radiations are evidently caused by the 
reflection of light from those flattened edges. The lateral ra- 
diations are produced by the light reflected from the edges of 
the lateral parts of the pupillary margin of the uvea, while its 
superior and inferior portions are covered by the eyelids. 
The whole uvea being hidden before the total close of the 
eyelids, these horizontal radiations vanish before the perpen- 
dicular ones. 
2. Some have inquired, Whence arises that luminous 
cross, which seems to proceed from the image of a candle in 
a looking-glass ? This is produced by the direction of the 
friction by which the glass is polished : the scratches placed 
in a horizontal direction, exhibiting the perpendicular part of 
the cross, and the vertical scratches the horizontal part, in a 
manner that may easily be conceived. 
3. Why do sparks appear to be emitted when the eye is 
rubbed or compressed in the dark ? This is Musschenbroek's 
