Manual of instructions for the guidance of army surgeons in testing the range and quality of vision of recruits, and in distinguishing the causes of defective vision in soldiers / by T. Longmore.
- Longmore, Sir Thomas, 1816-1895.
- Date:
- [1864]
Licence: Public Domain Mark
Credit: Manual of instructions for the guidance of army surgeons in testing the range and quality of vision of recruits, and in distinguishing the causes of defective vision in soldiers / by T. Longmore. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![from them are by these means rendered divergent; so much so that, the increased divergency being proportionate to the excessive convergent quality of the eye, they are brought to a focus upon the retina, and a proper image is then formed. Hence, no doubt, the oriscin of the vulgar name of the affection near-sightedness. The reason why objects more distant appear “ blurred,” or sur- rounded by a halo, is that after the rays pi*oceeding from them have been brought to a focus short of the retina, they again pro- ceed to diverge, and a certain j)roportion of these falling upon the retina, surround the picture formed before it. These scattered rays are called “ Circles of Dispersion.” The farther a given ob- ject is removed from the eye the greater will be the increase in the circles of dispersion. The habit which myopic persons have of partially closing their eyelids when looking intently at any object, is an instinctive endeavour to prevent some of the peri- ])heral rays of light reaching the eye, and so lessening the circles of dispersion upon the retina. Perhaps also at the same time the elongation of the eye-ball may be slightly lessened by means of the pressure then exerted upon it. Myopia, if severe, is not unfrequently found to be associated Avith Strabismus. The squint is occasionally, -but rarely, con- A^ergent, in cases Avhere the internal recti muscles have become disproportionately developed, or do not admit of being relaxed in proportion to the extei’nal muscles, from constant use, OAving to the approximation of objects for distinct vision, but is much more frequently found to be divergent. This prevalence of divergent squint with myopia is thus explained. As the myopic condition leads the patient to bring small objects near to the face to be seen clearly, so a strain is thrown upon the internal recti muscles, and as these get fatigued, they cease to act in true concert, and con- fused, or double vision, is liable to result. In this case one eye Avill turn outwards, so as to bring a less sensitive peripheral por- tion of the retina in line with the object, or, in other words, to make its image as indistinct as possible, so that the production of a mental impression by it may be avoided with more ease. The strabismus may be only temporary in its nature, occurring Avhen near objects are fixedly regarded, or it may be rendered permanent by continued repetition of the circumstances just explained.](https://iiif.wellcomecollection.org/image/b22333526_0029.jp2/full/800%2C/0/default.jpg)
