VISION. 



U63 



It is commonly supposed that a certain 

 visible conformation of the eye exists in con- 

 nection with, and characterises myopia ; that 

 the eye is full, the cornea prominent, the an- 

 terior chamber large, c. : but this is by no 

 means necessarily the case; for it may happen 

 that of two individuals in whom the apparent 

 configuration of the eyes is the same, one will 

 be myopic, the other presbyopic. It is also a 

 popular belief that near-sight decreases \\itli 

 age ; such does not accord with our experi- 

 ence. We have repeatedly seen persons above 

 sixty years of age as near-sighted as in their 

 youth, and are acquainted with a lady of 

 eighty-five, who has certainly never fatigued 

 her eyes with work or reading, and who still 

 uses, and has used for many years, a No. 7. 

 myopic glass. 



Myopia may arise from imperfection in the 

 form, consistence, or relation of some of the 

 refractive media ; or it may result from the 

 loss of adjusting power. The first description 

 may be induced by too great convexity either 

 of the cornea or lens, or both ; by an undue 

 density of any of the refractive media ; by a 

 too great distance between the cornea and 

 retina, &c. The effect is always the same, 

 namely, to cause the rays of light to be so 

 much refracted that, in place of being concen- 

 trated to a focus exactly on the retina a 

 point essential to distinct vision they are 

 brought to a focus before reaching it. It is a 



o O 



law in optics that the nearer an object is 

 brought to the eye, the more divergent are 

 the rays proceeding from it, and consequently 

 the greater the distance at which they will be 

 collected in a focus by refractive media. For 

 this reason it is that near-sighted persons 

 habitually hold objects very close to their 

 eyes, as by that means the image is thrown 

 back upon the retina. One of the causes of 

 imperfect vision in such persons, is the circle 

 of dissipation formed on the retina by the rays 

 after they have crossed at the focal point. 

 This is principally produced by the circum- 

 ferential rays ; and if these can be excluded, 

 vision is rendered much more distinct. Thus 

 it is that great assistance is afforded by look- 

 ing through a pin-hole in a card, and even 

 by holding to the eye a hollow roll of paper, 

 or the hand partially closed. The habit of 

 half shutting the lids which has given the 

 name to the affection, has reference to the 

 same object the exclusion of the circum- 

 ferential rays. 



In connection with this point, we may 

 mention that the reason stars are seen 

 earlier in the evening twilight with the assist- 

 ance of tubes than without, is, as pointed 

 out by Arago, that the tube cuts oft' a 

 large portion of the disturbing diffused liuht 

 of the atmospheric strata which intervene 

 between the star and the eye. In like 

 manner a tube is useful even in a dark 

 night in preventing the lateral impression of 

 the faint light which the particles of air re- 

 ceive from all the other stars in the sky, and 

 thereby increases the intensity of the lumi- 

 nous image and the apparent size of the star. 



Distant objects appear large to near-sighted 

 persons, because a distinct picture is formed 

 only at the point of intersection of the 

 rays proceeding from an object ; as this point 

 falls short of ilie retina in such persons, the 

 retina receives the rays beyond the point of 

 intersection, and they are consequently more 

 extended. M \opes also read with more ease 

 in partial darkness than those whose sight is 

 perfect. The quantity of rays from an object 

 is always in an inverse ratio to the square of 

 the distance ; and as these persons naturally 

 bring the object close to the eye, they receive 

 the full benefit of all the light proceeding from 

 it ; whereas other persons are obliged under 

 similar circumstances to close the lids and 

 contract the pupils to see distinctly, whereby 

 much less light enters their eyes than those 

 of myopic individuals ; hence the}' see with 

 greater effort and less distinctness. 



In congenital myopia the pupils are fre- 

 quently large, and do not contract fully. To 

 compensate for this the persons acquire the 

 h.ibit of knitting the brows and half closing 

 the lids, which gives them a characteristic 

 appearance. The handwriting of near-sighted 

 persons is generally small and cramped, the 

 proximity of the letters to the eyes increasng 

 the visual angle subtended by them, thus 

 increasing their apparent size. 



Among the exciting causes of acquired 

 myopia are, overwork of the eyes at the focus 

 for near objects, the indiscreet use of glasses, 

 and the necessity of working in obscure light. 

 The first and last may be c'assed together, 

 ami apply especially to engravers, watch- 

 makers, jewellers, and other artificers, the 

 nature of whose work requires close inspec- 

 tion. But there is yet another class espe- 

 cially subject to this impairment of vision, 

 namely, literary men. The public are little 

 aware of the extent to which the studious and 

 those who live by the exercise of their intel- 

 lect, suffer from imperfection of sight. Many 

 instances have fallen under our notice of poor 

 students and writers, whose poverty compelled 

 them to pursue their literary avocations in 

 the gloom of dusky apartments, or by the aid 

 of a dim candle, and who have become myopic, 

 and amblyopic in consequence. And scarcely 

 less numerous are those who are visited 

 with this affliction though pursuing their la- 

 bours under more favourable circumstances. 

 It would appear that even the study of oph- 

 thalmic science may cnuse the same penalty 

 to 1)6 paid ; for M. Desmarres informs us 

 that one of his pupils became very myopic by 

 exerting his eyes too much in the diagnosis 

 of diseases of the eye; a sad result of most 

 rare industry. 



A common cause of myopia is the improper 

 use of eye glasses. In this country they are 

 often worn from affectation ; but in France 

 glasses are used by the young men who wish 

 to escape the conscription, for the deliberate 

 purpose of rendering themselves near-sighted, 

 that constituting a ground for exemption 

 During the years 1831, 183.', and 1833, 7'.3 

 per 1000 of the conscripts examined in 



j A -1 



