528

OBSERVATIONS ON VISION.

tions are very justly attributed by Musschen-

' brock to the refraction of the moisture con-tiguous to the eyelids. But the lateral radia-tions are produced by the light reflected fromthe eyelashes.

2. Some have inquired, Whence arisesthat luminous cross, which seems to proceedfrom the image of a candle in a lodking-glass? This is produced by the direction ofthe friction by which the glass is commonlypolished : the scratches, placed in a horizon-tal direction, exhibiting the perpendicularpart of the cross, and tire vertical scratchesthe horizontal part, in a manner that mayeasily be conceived.

3 . Why do sparks appear to be emittedwhen the eye is rubbed or compressed in thedark ? This is Mussehenbroek’s fourth query.When a broadish pressure, as that of thelinger, is made on the opaque part of the eyein the daik, an orbicular spectrum appearson 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’shead, or of the nail, the image is narrowand bright. This is evidently occasioned bythe irritation of the retina at the part touched,referred by the mind to the place fromwhence light comingthrough the pupil wouldfall on this spot; the irritation is greatestwhere the flexure is greatest, that is, at thecircumference, and sometimes at the centre,of the depressed part. But in the presence oflight, whether the eye be open or closed, thecircumference 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 itfollows, that the tension and compression ofthe retina tend to destroy all the irritation,

except that whidh is pl'bducdd by its flexure;and this is so slight on the disc, that the ap-parent light there is fainter than that of therays arriving at all other parts through theeyelids. This experiment demonstrates atruth, which may be inferred from manyother arguments, that the supposed rectifica-tion of the inverted image on the retina doesnot depend on the direction of the incidentrays; since the mind can refer the object toits true relative situation without any assist-tance from tills direction. Newton, in hissixteenth quefy, has described this phantomas of pavonian colours, but I can distinguishno other than white ; and it seems most na-tural that this, being the compound of aver-age- of all existing sensations of light, shouldbe produced when nothing determines to anyparticular colour. 1 This average seems to re-semble the middle form, which Sir JoshuaReynolds has elegantly insisted on in his dis-courses ; so that perhaps some principles ofbeautiful contrast of colours may be drawnfrom hence, it being probable that those co-lours which together approach near to whitelight 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 ofpressure has ceased, so that the cause of theirritation of the retina is a change, and not adifference of form ; and therefore the sensa-tion of light appears to depend immediatelyon 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 ofpain, and a continued pressure he then madeon the sclerotica, while an interrupted pres-sure is made on the cornea; we shall fre-quently he able to observe an appearance of