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.

Date:: 1875

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 Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

32/148 page 18

$To jSnd the correcting lens.—The excess of converging- power having been determined, it is corrected by a lens of corre- sponding diverging power. In the example given a —iV, oi a 15 concave lens, will be the correcting lens, because this will neutralize the + in excess. The distant point of distinct vision of the eye in this example when no lens is placed before it will be 15 inches off; or, in other words, the rays of nearest approach to parallel rays which the eye is able to focus with accuracy are the rays with that degree of divergency which they have when they start from a point placed at a distance of 15 inches from the eye. Alens held in front of the eye causes parallel rays from distant objects to have the same degree of divergency, and thus the eye is rendered competent to form distinct retinal images of those objects, just as it was able to do of the nearer objects at 15 without the lens. Mem.—In defining the degree of M. with precision, a sKght correction has to be made for the distance at which the trial lens is placed from the eye; and when lenses are placed before both eyes together, another correction becomes necessary in practice for the gain in refraction due to the amount of accommodation in activity which is associated with the convergence of the optic axes when both eyes are employed in regarding an object. Rather of Emmetropia, Myopia, and Hypermeti'opia, and of tlie degrees in whicli the two latter conditions exist; because medical ojficers cannot usually avail them- selves of regular series of lenses for conducting such investigations ; because the mode of observation can be easily learned and the obsen'ations can be conducted within a moderate range of distance, such as can be obtained in any ordinary room ; and further, since the trials are usually made on persons who have no knowledge of the effects of lenses, because efforts at deception, if attempted to be practised, are more readily defeated. Other methods of carrying out the investigation are only briefly referred to in the text as the main object is to render this manual for the use of beginners as concise and simple as possible. Occasional cases will occur, when atropine has not been employed, in which the true degree of myopia may not be exactly found by the use of the -j- ]0 lens, because the person under trial has not been able altogether to relax his accommodation when fixing his sight on an object within 10 inches distance; but such instances are very rare when the trial is thoroughly con« ducted. This is proved by the fact that in almost all cases the degree of myopia thus found is shown to be the true one by the corresponding concave lens correcting the vision for distant objects.$