There are several curious phenomena connected with the subject we are treating of, some of which we will briefly allude to. If a piece of white paper is viewed through two different coloured glasses, held one to each eye (for instance, through a blue and a yellow glass), the paper is not seen of a green colour, or rather of the whitish colour which would result from the strict mixture of the two colours, but in part blue and in part yellow. Sometimes one colour predominates, sometimes the other ; and If the experiment is long continued, the mingling of tho colours, to which there was at first no tendency, becomes more evident. Similar phenomena are observed when two dissimilar pictures are viewed in the stereoscope, and it does not appear to be in the power of the will to determine the appearance of either picture or either colour. These facts seem to show that the two eyes are not always in action together ; but that at one time the sensations of one eye pre tbfnalnate, and at another those of the other. If the eyes are closed after being fixed for some time on en object, we still continue to see its image, and the duration of this image, or " spoctrum,"as it is called, Is In a direct ratio with the impression which caused it.
Spectra left by the images of white or luminous objects are ordi narily white or luminous, and those left by dark objects dark ; but if, instead of closing the eyes, they are directed upon some white surface, as a sheet of paper, the colours of the spectra are reversed, those which were white when the eyes were closed becoming black when directed to the white surface, and rice rerod. These phenomena are easily ex plained. The part of the retina which has received the luminous image remains for some time afterwards in an excited state, while that which has received a dark image is in an unexcited and therefore much more excitable condition. When the eye in this condition is directed
towards a white surface, the luminous rays from this surface produce upon the excited parts of the retina a much more feeble impression than upon the unexcited, and the latter consequently appear more illuminated. Spectra are sometimes coloured, although the objects which excited them are colourless ; such is the case if the impressions on the retina are very intense, as when produced by the sun's image. But the most curious phenomena relative to ocular spectra arise from the impression of coloured objects on the retina ; the spectra con sequent on these, although coloured, are not of the same colour as the object, but the opposite or complementary colour. Thus the spectrum of a red object is green, that of a green object red, that of a violet yellow, &c. There are two modes of explaining these phenomena, the !mat objectionable of which is the following, offered by Mailer " The perception of any one of the three simple colours consists merely in the retina being in one of those conditions to which it has a tendency when in a state of excitement ; if this condition be artificially excited in an intense degree, the retina acquires an extreme tendency to that of the complementary colour, which consequently is perceived as the ocular spectrum?' The disappearance of images which fall on the retina at the entrance of the optic nerve, the luminous circles seen on making pressure with the finger on the globe of the eye, and the vascular network which, under certain circumstances, we perecive in our own retina, have already been alluded to and their causes explained in the article EYE, in NAT. HIST. Drv.