Blindness may be due to many causes, loss of transparency of the cornea (p. 480) or lens (p. 482) preventing the entrance of light, diseases of the chervil, retina, or optic nerve (p. 483), affections of the brain, &c.
Amaurosis and Amblyopia are old words, used to express, the former total blindness, and the latter dimness of sight. They were used before the causes of such conditions could be ascertained ; but as these conditions are now referred to affections of retina, optic nerve, &c., the old names are being dropped.
Partial Blindness (Ilenziopia) of an eye, with which nothing seems wrong, may occur. The person, looking straight at someone stand ing in front of him, sees only one half of him. Half of the retina is blind. This may begin in one eye, but extend to both, the vision.of a corresponding half of each retina being lost. It is due to disease of the optic nerve roots. Syphilis is frequently the cause.
Night-blindness (llemeralopia)in the result of a blunting of the sensitiveness of the retina owing to some excessive irritation. It has its origin in the long-continued action of strong light upon the eye, and is also connected with weakness of the nervous system. The retina may be blunted by the strong glare from snow; and this may occur to those who walk over fields of snow or ice among the mountains, without having the eyes protected by a dark veil or dark glasses. This is called snow-blindness. Simi larly, moon-blindness is a like condition, pro duced among sailors in the tropics who sleep on deck under the full light of the moon.
Colour-blindness (Daltonism).—This affec tion was called daltonism because it was first described by John Dalton, in 1794. Colour blindness, however, exists in several forms. The form to which the term " daltonistu " is properly applied is a lack of sensibility to red. Red light and red objects appear greenish or grayish. Since, according to the theory ex plained on p. 459, the sensation of colour is a mixed sensation, dependent upon the excite of three sets of nerve fibres, one set gen-. sitive to red, another to green, and a third to violet, an excitement of all three in a definite proportion producing the sensation of white, and in various other proportions the sensations of the other colours,—since this is so, the loss of sensibility to red will affect not only the perception of red but all other perceptions in which the sensibility to red has any great place.
Thus the sensation of yellow is due to moderate excitement of the fibres sensitive to red and green, and feeble excitement of the fibres sen sitive to violet. Consequently to the person not at all sensitive to red the whole character of the sensation is altered. The fibres sensitive to green are moderately excited, there is no response to the red, and the violet is feebly excited. Thus yellow appears a decided green; red is confounded with brown and green; violet is called blue, and rose-colour is not dis tinguished from blue.
Besides insensibility to red there is a similar lack of perception of green—green-blindness. Blue-blindness is another form, in which blue and green, and blue and yellow are confounded.
Red-blindness is the most common. It is in herited, as a rule, and dates from birth, and is commoner in men than women. The best test is to give the person a handful of variously coloured wools and ask him to place all of one colour together.
In rare cases colour-blindness is complete, and the person recognizes only blacks and whites.
Double Vision is the result of paralysis of some of the muscles of the eyeball. The affected eye cannot be moved round in a par ticular direction as far as its neighbour ; the correspondence between the two eyes is there fore lost, and at that point the object looked at appears double. In other directions of the eyeballs the vision is single. It is the result of tumour or other affection of the brain, such as arises in the course of syphilis. Other causes also may induce it. Accompanying it there are probably other signs of paralysis, droop of the eyelid, dilated pupil, &c., and perhaps some paralysis of the face.