Frames, of whatever material they may be formed (and blue or bronze steel is the best), should possess the following qualifications. The rims should accurately tit the form of the lenses, and be sufficiently strong to re tain them ;, the connecting arch or bridge must be of such width and shape that it will fit the nose accurately, and maintain the centre of each glass in front of the axis of the corresponding eye. This is a point of great importance, otherwise the eyes will not have the full :assistance of the lenses. The necessity for the bridge being adapted correctly was proved by a case that re cently came under our notice, where a wart on the side of the nose had degenerated into a scirrhous growth from the friction of an ill fitting spectacle frame which constantly pressed upon, and irritated that point. The lateral branches of the frame should be suffi ciently strong and elastic to retain it in its place, firmly and immoveably under all move ments of the head ; and they should be of such a shape and size that no undue pressure can be caused crn the temples or sides of the head. Besides the injury to the eyes from ill-fitting frames, persons acquire the habit of grimacing in their efforts to see through them. Hand glasses and those retained on the nose by a spring are objectionable ; the latter, when clipping the nose, almost invariably throw the centres of the glasses out of the axial lines of the eyes, and both are deficient in that steadiness which is indispensable to perfect and comfortable vision.
The power of biconvex glasses is indicated by their numbers, and these numbers signify the inches of the focal length.
The numbering of the French glasses, whether presbyopic or myopic,, has a much more extensive range than the British, as will be seen by the following lists.
Presbyopic (French). 80, 72, 60, 48, 36, 30, 24, 20, 18, 72, 60, 48, 36, 30, 24, 20, 18, 16, 15, 14, 13, 12, 1 1, 10, 9, 8, 7, 6, 5, 41, 4, 31, 3, 21, 2, 11, 1 Z, 1.
Myopic (French). 60, 30, 20. 18, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 41, 4, 31, 3,1, 3, 21, 21, 2, It, 11, 1-k, 1.
Presiyopic (British). 48, 36, 30, 24, 20, 18, 14, 12, 10, 9, 8, 7, 6, 51, 5, 41, 1, 3:, 21, 2t, 2i, 2, 11, 11.
Myopic (British). 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20.
The numbers of the double convex glasses below 5 in both scales are confined to patients who have undergone the operation for cataract.
The golden rule in the selection of spec tacles, whether for myopia or presbyopia, is to choose the lowest power that is productive of distinct vision ; and so long as these afford the necessary assistance the party should rest contented, for if he begins with too high a number or rashly increases the power, it will be found not only difficult to go back, but equally so to subdue the inclination for further increase.
A myopic person should therefore select glasses which will enable him to distinguish the outlines of objects distinctly at about forty-five feet, without diminishing them in the slightest degree ; and with these he should be satisfied, not changing them for a higher power unless absolutely obliged.
It frequently happens that the eyes of myopic persons are of cdifferent focal length, in which case great care should be taken by diligent trial to determine the powers which will bring the eyes to perfect equality, and with these lenses the frames should be fitted ; but there is another condition of vision which is sometimes embarrassing ; it is when one eye is myopic and the other presbyopic. This can only be ascertained by careful trial of each eye with convex and concave lenses, and the results are sometimes quite unex pected ; if therefore a person using glasses complains of' faulty vision with one eye, which cannot be satisfactory accounted for, it proper to try the eye with a different descrip tion of lens, which will frequently show the nature of the case at once. Eyes thus faulty require a double convex and a double concave lens fitted into the same frame, and the same rules apply to their selection, as to glasses generally.
In England the lowest power in use for presbyopia is a glass of 48 inch focus; in France it is otherwise, as will be seen by reference to the table already given. M. Sichel commences with a 72 inch, and in some cases with a 96 inch; Mr. A. Ross, however, who has had an im mense amount of experience as an optician, informs us that he has met with but one per son who could perceive any sensible difference between these two powers, as far as assistance to the sight was concerned. It is quite pos sible that in the early stage of presbyopia a 72 inch class may be sufficient, and if found to be so, it should by all means be preferred to a higher number ; but, practically speaking, a '48 inch is that most usually required, because persons in this country seldom seek assistance until the presbyopia has advanced beyond the aid of a 72 inch glass. It has been recom mended to calculate the requirements of the eye by the age of the individual, but this is fallacious, as eyes differ much in their natural powers of vision, and not less in the amount of assistance they need. The only true mode of obtaining suitable spectacles is by absolute trial.
Persons should not be satisfied with a hasty trial of glasses in an optician's shop, for the eyes soon become excited and confused, rendering it impossible to arrive at a correct decision. It is for better to select two or three pairs of spectacles which are near the mark, and to test them leisurely at home. Those•to be selected should simply give black ness and distinctness to the letters of a book, and enable a person to read or work at the natural distance with perfect comfort ; such glasses supply to the eyes precisely that amount of refractive power in which they are deficient.