272 SENSE OP SIGHT. 



degrees ; and hence it is impossible to say, a priori, what is the precise 

 lens, that will suit any particular individual. This must be decided 

 by trial. The opticians have their spectacles arbitrarily numbered to 

 suit different periods of life; but each person should select for himself 

 such as will enable him to read without effort at the usual distance. 

 A degree of myopy may be brought on by long-protracted attention 

 to minute and near objects, as we observe occasionally in the watch- 

 maker and engraver; and, again, a person who has been long in the 

 habit of looking out for distant objects, as the sailor, or the watchman 

 at signal stations, is rendered less fitted for minute and near inspec- 

 tion. During the domination of Napoleon, when the conscript laws 

 were so oppressive, the young men frequently induced a myopic state, 

 by the constant use of glasses of considerable concavity; the defect 

 being esteemed a sufficient ground of exemption from military service. 



Another subject, which has given rise to much disputation and ex- 

 periment, is, why, as we have two eyes, and the image of an object is 

 impressed upon each of them, we do not see such object double ? 

 Smith 1 and Buffon 2 consider, that in infancy we do see it so; and that 

 it is not until we have learned by experience, by the sense of touch 

 for example, that one object only exists, that we acquire the power of 

 single vision. After the mind has thus become instructed of its error, 

 a habit of rectification is attained, until it is ultimately effected uncon- 

 sciously. The objections to this hypothesis are many and cogent. 

 We are not aware of any instance on record, jn which double vision 

 has been observed in those, who, having laboured under cataract from 

 birth, have received their sight by an operation; and we are obviously 

 precluded from knowing the state of vision in the infant, although the 

 simultaneous and parallel motions of the eyes, which are manifestly 

 instinctive, and not dependent upon habit, would induce us to pre- 

 sume, that the images of objects as soon as the parts have attained 

 the necessary degree of developement are made to fall upon corre- 

 sponding points of the retina. It may, also, be remarked, in favour 

 of the instinctive nature of this parallel motion of the eyes, that in the 

 blind, although we may find much irregularity in the motions of the 

 eyeball, owing to no necessity existing for the eyes being directed to 

 any particular point, the eyeballs move together, unless some de- 

 ranging influence is exerted. The truth is, as we have already ob- 

 served, the encephalon is compelled to receive the impression as it is 

 conveyed to it ; and even in cases, in which we are aware of an illu- 

 sion, the perception of the illusion still exists in spite of all experience. 

 If the finger be pressed on one side of the eyeball, an object, seen in 

 front, will appear double, and the perception of two objects will be 

 made by the brain, although we know from experience that one only 

 exists. This occurs in all the various optical illusions to be presently 

 mentioned. 



The effect of intoxication has been adduced in favour of this hypo- 

 thesis. It is said that in these cases the usual train of mental asso- 

 ciation is broken in upon, and hence double vision results. The proper 



1 Optics, Cambridge, 1738. 3 Memoir, de 1'Academ. des Sciences, 1743. 



