324 PHILOSOPHICAL TRANSACTIONS. [aNNO l/QS. 



horizontal direction exhibiting the perpendicular part of the cross, and the ver- 

 tical 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 4th query. When a broadish pressure, 

 as that of the finger, is made on the opaque part of the eye in the dark, an or- 

 bicular spectrum appears on the part opposite to that which is pressed: the light 

 of the disc is faint, that of the circumference much stronger ; but when a narrow 

 surface is applied, as that of a pin's head, or of the nail, the image is narrow and 

 bright. This is evidently occasioned by the irritation of the retina at the part 

 touched, referred by the mind to the place from whence light coming through 

 the pupil would fall on this spot; the irritation is greatest where the flexure is 

 greatest, viz. at the circumference, and sometimes at the centre, of the depressed 

 part. But in the presence of light, whether the eye be open or closed, the cir- 

 cumference only will be luminous, and the disc dark; and if the eye be viewing 

 any object at the part where the image appears, that object will be totally invi- 

 sible. Hence it follows, that the tension and compression of the retina destroys 

 all the irritation, except that which is produced by its flexure; and this is so 

 slight on the disc, that the apparent light there is fainter than that of the rays 

 arriving at all other parts through the eye-lids. This experiment demonstrates a 

 truth, which may be inferred from many other arguments, and is indeed almost 

 an axiom, viz. that the supposed rectification of the inverted image on the retina 

 does not depend on the direction of the incident rays. Newton, in his l6th 

 query, has described this phantom as of pavonian colours, but I can distinguish 

 no other than white; and it seems most natural that this, being the compound 

 or average of all existing sensations of light, should be produced when nothing 

 determines to any particular colour. This average seems to resemble the middle 

 form, which Sir Joshua Reynolds has elegantly insisted on in his discourses; so 

 that perhaps some principles of beautiful contrast of colours may be drawn from 

 hence, it being probable that those colours which together approach near to 

 white light will have the most pleasing effect in apposition. It must be observed, 

 that the sensation of light from pressure of the eye subsides almost instantly 

 after the motion of pressure has ceased, so that the cause of the irritation of the 

 retina is a change, and not a difference, of form; and therefore the sensation of 

 light appears to depend immediately on a minute motion of some part of the 

 optic nerve. 



If the anterior part of the eye be repeatedly pressed, so as to occasion some 

 degree of pain, and a continued pressure be then made on the sclerotica, while 

 an interrupted pressure is made on the cornea; we shall frequently be able to ob- 

 serve an appearance of luminous lines, branched, and somewhat connected with 



