This experiment of Scheiner led Dr. Porter field to invent an instrument called an op ,tometer, for the determination of the focal distance of the eye ; and Dr. Young sub sequently greatly improved upon it, his in strument being simple in construction, and both convenient and accurate in its applica tion.
The greatest distance of human vision is so variable, that no arbitrary limits can be as signed to it. Uncivilised tribes, as the North American Indians, the inhabitants of the im mense Asiatic steppes, and the New Zealan ders, possess powers of sight which are almost incredible. It is interesting, however, to remark, that the mean degree of capability of vision was the same among the ancient Greeks and Romans as at the present
From the experiments of Harris, it seems that a simple object, as a black square on a white ground, or a white square on a black ground, can be seen under a less angle than the equal parts of a compound object, as the squares of a chequered figure, and that their least, or minimum visible angle cannot be less than 40": others, however, say 30". If it is 40", the size of the image on the retina will be Tolz-5 inch. At a medium, Harris thinks it not less than 2'. He remarks that the diffi culty of keeping the eye perfectly steady, may be one cause why a single object can he dis cerned under a less angle than the parts of a complex one ; and that it is natural to sup pose that the fewer objects we contemplate, and the more they differ in colour, the easier we can distinguish their several impressions on the retina. The result of repeated and very careful experiments by Hueck, tends to show that white objects on a black ground are seen at a greater distance than black objects on a white ground*, and this is fully corroborated by an instance mentioned by Humboldt. This distinguished traveller was with Bonpland, at Chillo, near Quito, in South America, from whence the long extended ridge of the volcano of Pichincha is visible. Bonpland had pro ceeded on an expedition to the volcano, and Humboldt, with others, was somewhat anxiously looking out for him. The Indians of the party recognised the traveller as a white point moving along the face of a black basaltic precipice, before he was discovered by Humboldt and others, who were looking out for him with telescopes (a proof, by the way, of their excellent vision) ; but in a short time the Europeans also were able to distinguish the white moving figure with the naked eye.
The weather was clear, and the distance 14.8 geographical miles. A small object in motion, however, is more easily discerned than if at rest : by the gradual motion of the image over 'the retina, the impression upon each part con tinuing for some time, the effect is the same as if a linear image were formed.
Duration of impressions. — A beautiful pro 'vision to insure uninterrupted vision, is afforded by the duration of impressions upon the retina, whereby we never lose sight of an object we are viewing, during the necessary winking of the eyelids. M. D'Arcy found that the light of a live coal, moving in a circle, at the distance of 165 feet, maintained its impression upon the retina somewhat more than the seventh part of a second. From the observations of Plateau, it seems that the interval of time, during which the impression retains the same intensity, is more consider able in proportion as the light is moderated ; the mean duration of the impression excited by all the colours from the instant of their maximum intensity till their entire disap pearance, being one-third of a second in a dark room, and one-sixth of a second in a light room. If two or more impressions suc ceed each other at such short intervals that the first has not faded away before the next commences, they run one into another, the eye seeming to receive but a single impression ; as, for example, the appearance of a circle of fire from a revolving burning stick — when it is in reality the combination of many indi vidual impressions succeeding each other with rapidity : upon this depends many of the most beautiful examples of the pyrotechnic art ; from the same cause a flash of lightning ap pears as a continuous line of light, because the light emitted at any point of the line remains upon the retina until the cause of the light passes over the succeeding points. In order that an object may become visible, it is necessary that the retina should be exposed to 'its influence for an appreciable time, a fact first pointed out by Lord Bacon, who observes, that notwithstanding the rapidity of the act of vision, a certain time is required for its ex ercise, which is proved by certain objects, as, for instance, a musket-ball, being invisible on account of the velocity of their motion ; for the flight of the ball, he remarks, is too swift to allow an impression of its figure to be con veyed to the sight. This subject has been recently investigated with much success by Mr. William Swan*, who arrives at the fol lowing conclusions.