^2« 



OBSEHVATlbNS ON VISION. 



tions are ver}' jusdy attributed by Musschen- 

 broek to the refraction of the moisture con- 

 tiguous to the eyelids. But the lateral radia- 

 tions are produced by the light reflected from 

 the eyelashes. 



2. Some have inquired. Whence arises 

 tliat luminous cross, which seems to proceed 

 from the image of a candle in a lo6king- 

 glasi? This is produced by the direction of 

 the friction by which the glass is commonly 

 polished : the scratches, placed in a horizon- 

 tal direction, exhibiting the perpendicular 

 part of ihe 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 Mnsschenbroek's fourth query. 

 When a hroadish pressure, as liiat of the 

 finger, is made on the opaque part of the eye 

 in the dark, an orbicular spectrum appears 

 on the part opposite to that which is pressed : 

 the light of the disc is faint, that of the cir- 

 cumference much stronger; but when a nar- 

 row 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, that is, 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 

 circumference only will be luminous, and the 

 disc dark ; and if the eye be viewing any ob- 

 ject at the part where the image appears, 

 that object will be almost invisible. Hence it 

 follows, that the tension and compression of 

 the retina tend to destroy all the irritation. 



except that which is produced by its flexure; 

 and this is so slight on the disc, that the ap- 

 parent light there is faiHter than that of the 

 rays 'arriving at all other parts through the 

 eyelids. This experiment demonstrates a 

 truth, which may be inferred from many 

 other argtiments, that the sapp6sed rectifica- 

 tion of the inverted image on the retina does 

 not depend on the direction of the incident 

 rays; since th6 mind can refer the object to 

 its true relative situation without any a«sist- 

 tance from this direction. Newton, in his 

 sixteenth query, has described this phantom 

 as of pavonian colours, but I ean distinguish 

 no other than white ; and it seems most na- 

 tural that this, being the compound or aver- 

 age of all existing sensations of light, should 

 be produced when nothing determines to any 

 particular colour. This average seems to re- 

 semble the middle form, which Sir Joshua 

 Reynolds has elegantly insisted on in his dis- 

 courses ; so that perhaps some principles of 

 beautiful contrast of colours may be drawn 

 from hence, it being probable that those co- 

 lours which together approach near to white 

 light will have the most pleasing effect in ap- 

 position. It must be observed, that the sen- 

 sation of light, from pressure of the eye, sub- 

 sides 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 sensa- 

 tion of light appears to depend immediately 

 on a minute motion of some part of the op- 

 tic nerve. 



If the anterior part of the eye be repeat- 

 edly pressed, so as to occasion some degree of 

 pain, and a continued pressure be then made 

 on the sclerotica, while an interrupted pres-* 

 sure is made on the cornea ; we shall fre- 

 quently be able to observe an appearance of 



