Spectacles Or Eyeglasses (Hints On The Use Of)
( Originally Published Early 1900's )
From a valuable little book, "Practical Remarks on Impaired Vision," by Mr. Cooper, the well-known London optician, we extract the following : "It cannot be too strongly urged upon anyone about to use spectacles for the first time that that power which will enable him to read without much exertion by candle-light is the only power suitable for him. It is by candlelight only that he should use glasses at first, and as soon as he finds that he stands in need of glasses by day as well as by candle-light, and that the glasses he uses no longer afford him sufficient assistance by candlelight, it will be proper to use the next power for the evening, but for the evening only. and to al-low himself the use of the others and their use only—during the day.
"The greatest caution as to increasing the power of glasses should be observed ; for persons who change their glasses, unnecessarily increasing their power each time, are exhausting the resources of art, instead of economizing them as much as possible. Optical aid call only be extended to a certain point, and the steps to that point should be as slow and as numerous as possible. By exercising prudent precautions, persons may often attain great age, and yet never require the aid of glasses beyond a very moderate power ; others, on the contrary, who from ignorance frequently increase the power of their glasses, may run through the whole assortment, and leave themselves only the most inconvenient resources to fall back upon —viz., the very highest powers."
Common, cheap spectacles sometimes appear to answer as well as those which cost three or four times as much ; but cheap glasses are not to be depended upon ; they are sometimes ground irregularly and imperfectly, and then they injure the eyes. It is better, therefore, to have spectacles from a respectable optician, who has a character to maintain. Spectacles having lenses called pebbles, which is rock crystal, are not liable to be scratched like glass ; but they are not in any degree better than those of glass for the eyesight ; and if care be taken of the latter they do just as well.
There are three kinds of spectacle glasses, the convex, the concave, and the periscopic. The first are to correct short sight, the second to correct long sight, while the periscopic are for either. This last description of lens is both concave and convex, the former on the side nearest the eye, the latter on the side furthest from it. For long sight, as well as short, the convexity and concavity are made to differ so as to furnish any required focus. It is to be observed that in glasses of this form the aberration of light is greater than in any other lenses, and that periscopic glasses are liable to be scratched. They have, however, one very great advantage, which is this : With common glasses, especially concave, the wearer can see only through the exact middle of the lens ; he must, therefore, turn his head whenever he directs his view to any lateral object. With periscopic glasses he may see through any part of them, and can observe objects by his side without turning his head. If, however, periscopic glasses be defectively made, they are injurious to the eyes. They should be gauged and examined before they are purchased, to be assured of their accuracy.