1444 



VISION. 



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 



corroborates Colonel Smith's results: hence it ap- 

 pears that the liability of brown is ^~-6, and 

 that red is by far the most fatal colour, green next, 

 then brown, and Austrian grey the least. 



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. 



When the eye receives a succession of 

 flashes of equal duration from a light of con- 

 stant intensity, which succeed each other so 

 rapidly as to* produce a uniform impression, 

 the intensity of this aggregate impression will 

 also be constant, provided the number of 

 flashes in a given time varies inversely with 

 the duration of each. The brightness of the 

 impression produced by flashes of light of a 

 given intensit}', which succeed each other so 

 rapidly as to produce a uniform impression 

 on the eye, is proportional to the number of 

 flashes in a given time. 



When light of a given intensity acts on the 

 eye for a short space of time, the brightness 

 of the luminous impression on the retina is 

 exactly proportional to the time during which 

 the light continues to act. This law has been 

 proved to be true for impressions lasting from 

 Tirfe to^ofasecond. 



The intensity of the impression produced 

 by light, which acts on the eye for T i^ of a 

 second, is almost exactly T Vh of the apparent 

 brightness of the light when seen by uninter- 

 rupted vision ; and the time required for light 

 to produce its full effect on the eye seems to 

 be about -^ of a second. Lights of different 

 intensities produce their complete impressions 

 on the eye in equal times, so that the light of 

 the sun requires the same time as common 

 artificial light to produce its impression on 

 the eye. The brightness of an impression on 

 the eye increases with a rapidity exactly pro- 

 portional to that of the light by which it is 

 produced. 



Rays of different refrangibility act on the 

 eye with equal rapidity. The apparent bright- 

 ness of the spark produced by electricity of 

 high tension is only about TouVo^ f ^' 1a * 

 its apparent brightness would become if its 

 duration were prolonged to T \y of a second ; 

 and the brightness of electric light increases 

 with the tension of the electricity. 



The wonderful rapidity of the electric light is 

 shown by the following experiments : View ed 

 by the illumination of an electric spark, the 

 spokes of a wheel in the most rapid rotation 

 appear stationary, vibrating cords seem to be 

 in a state of repose, and a succession of drops, 

 which generally appear to the eye as a con- 

 nected stream, is seen to be but a succession 

 of drops, because the impression of each 

 image lasts for so short a time that the posi- 

 tion of the moving bodies is not altered. The 

 light of electricity of high tension has a less 

 duration than the millionth part of a second. 



When on the subject of light, we have 



alluded to the reproduction of the impression 



" of white light by the rapid revolution of a disk 



* Transactions of the Royal Society of Edinburgh, 

 vol. xvi. p. 581. 



