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 off a large portion of the disturbing diffused light 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 Inini 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 of ; as this point falls short of the retina in such persons, the retina receives the rays beyond the point of intersection, and they are consequently more extended. 111yope& 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 they 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 habit 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 classed together, and 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 cause the same penalty to he 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, 1832, and 1833, 7..3 per 1000 of the conscripts examined in France were rejected on the score of short sight.
Oculo-cerebral congestion may give rise to myopia ; a case alluded to by Smith was probably of this nature, — that of it person who suddenly became myopic on coining out of a cold bath. And Revielle- Parise men tions an officer who was similarly attacked at the end of a troublesome ague. Local con gestion is said to produce the same effect. Desmarres relates a remarkable instance of a lady excessively presbyopic, who became near sighted during a severe attack of conjunc tivitis, the former condition of vision returning after the attack had subsided. Mr. Tyrrell also mentions a lady who had long suffered from granular lids, which were eventually cured, but she afterwards required the aid of concave glasses, from the cornea having become un usually convex during the continuance of the chronic disease. In each of these cases, there may have been some change, either in the quantity or quality of the aqueous humour, or in the consistence of the cornea.