VISION. 



1467 



must be of such width and shape that it will 

 fit the nose accurately, and nv.iintaiu 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 on 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 hitter, 

 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 bi-convex 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. 



Prcsbifopic (French). 80, 12, 60, 48, 36, 30, 

 24., 20, '18, 72, 60, 48, 36, 30, 24, 20, 18, 16, 

 15, 14, 13, 12, 11, 10, 9, 8, 7,6, 5, 4.1, 4, 

 3*-, 3, 21, 2, H, H, 1. 



Myopic (French). 60, 30, 20, 18, 16, 15, 14, 



13, 12, 1 1, 10, 9, 8, 7, 6, 5, 4-i, 4, 83, 3$, 3, 

 2f,2i,2, If, U, li, 1. 



Presbyopic (British'). 48, 36, 30, 24, 20, 18, 



14, 12, 10, 9, 8, 7, 6, 5J, 5, 41, 4, 3|, 2J, 2f, 

 2' , 2- 2 -, 2, 1-, I 1 -. 



Myopic' (SritisK). 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 lie 

 should be satisfied, not changing them for a 

 higher power unless absolutely obliged. 



It frequently happens that the eyes of 



myopic persons are of different 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 Irames should be fitted; 

 but there is another condition of vision which 

 is sometimes embarrassing ; it is when one 

 eye is myopic anil 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 fault}' vision with one eye, which 

 cannot be satisfactory accounted (or, it is 

 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 glass 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 far 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. 



Cylindrical Eye. This peculiarity of the 

 organ of vision was first investigated and ex- 

 plained by the present Astronomer Royal. It 

 depends on the curvature of the cornea being 

 greater in the vertical plane than the hori- 

 zontal, whereby the rays are refracted to a 

 nearer focus in a vertical than in a horizontal 

 plane ; this gives rise to much confusion of 



