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Short-sighted Person who has had occasion to increase the depth of his Glasses, if he began to use them in the form of Spectacles; whereas he can recollect several instances, where those have been obliged to change their Concave Glasses repeatedly, for others of higher powers, who had been accustomed to apply them to One Eye only." - Dr. W. C. WELLS on Vision,
The advantage of a pair of Spectacles, over a Single Glass, is sufficiently obvious Objects appear brighter when seen with Both Eyes, than they do when viewed with One only. See Appendix.
FOR THE CHOICE OF SPECTACLES FOR SHORT-SIGHTED PERSONS, I have few Rules to offer-it is a defect which has no reference to Age-no stated progression that can be a foundation to guide an Optician, or lead him to recommend one Glass in preference to another — but all depends on the observation of the Short-sighted themselves— who I most earnestly advise, to be content with as Shallow Concaves as possible i. e. to take the least Concave Glass through which they can distinctly discern the names
on the corners of the Streets, and which gives a decided outline to objects whose distance does not exceed about 40 feet, and which renders them clear, without making Vision dazzling and glaring-the Glass which does, is too deep by a Number.-See Appendix.
After your Eyes have been long accustomed to the assistance of Concave Glasses, the smallest variation in the degree of their Concavity will be extremely distressing and injurious to the Sight: when you have found Glasses which exactly suit you have Two or Three pairs fitted to your Spectacles, that you may be provided if a Glass gets scratched or broken.
Near-Sightedness generally continues the same during Life, and precisely the same Glass continues to afford precisely the same assistance.
After persons have used the same glass for some years and it is broken, &c. it is often extremely difficult to make them think, that any new one suits their Sight exactly so well as the Old one which they had been in the habit of long using: therefore Pebbles are especially desirable for the Short-sighted.
Persons who are extremely Short-sighted, to prevent their stooping in writing-to read
Music, &c. &c. may wear Spectacles with very shallow Concaves, just enough to enable them to see such objects at the same distance which others do.
A Deeper Concave is wanted to see very Distant Objects. Dr. MASKELYNE,* the late Astronomer Royal, to look at the Constellations, found it convenient to use a Concave
✦ "When I look at the brighter fixed stars, at considerable elevations, through a concave glass fitted, as I am short-sighted, to shew them with most distinctness, they appear to me without scintillation, and as a small round circle of fire of a sensible magnitude. If I look at them without the concave glass, or with one not suited to my eye, they appear to cast out rays of a determinate figure not exactly the same in both eyes, somewhat like branches of trees (which doubtless arise from something in the construction of the Eye), and to scintillate a little, if the air be not very clear. To see day objects with most distinctness, I require a less concave lens by one degree than for seeing the stars best by night, the cause of which seems to be, that the bottom of the Eye being illuminated by the day objects, and thereby rendered a light ground, obscures the fainter colours blue, indigo, and violet, in the circle of dissipation, and therefore the best image of the object will be found in the focus of the bright yellow rays, and not in that of the mean refrangible ones, or the dark green, agree
one degree deeper than he wore for common purposes in the day time- and the Author uses No. 2 in ordinary, but at Night sees many faint Stars well with No. 3, which he cannot see at all with No. 2. This is especially remarkable in the early part of the Evening when the Stars first become visible. And at Large Theatres, he finds one Number deeper than that which he ordinarily uses is a very advantageous indulgence to his Eye.
To give more assistance to the Sight to see a distant object, many persons hold a Concave Eye-Glass very obliquely to the Eye-in which position, a Concave No. 3 will give almost the same sharpness of outline to objects as No. 3 when held parallel to the Eye.
FOR DISTANT OBJECTS, extremely Shortsighted persons should use A SMALL OPERAGLASS, which having an adjustable focus, if it
able to Newton's remark, and consequently nearer the retina of a short-sighted person; but the parts of the retina surrounding the circle of dissipation of a star being in the dark, the fainter colours, blue, indigo, and violet, will have some share in forming the image, and consequently the focus will be shorter."-See Phil. Trans. Vol. LXXIX.
only magnifies Twice,* will be infinitely better than any single Concave, because it can be exactly adapted to various distances.
My "Invisible Opera-Glass," a contrivance of Mr. PIERCE and myself, is a great acquisition to Short-sighted Persons, and is an inestimable little Instrument for Artists, &c. who wish to discern the distinct outline of objects at short inaccessible distances; i. e. for an Architect to see the exact outline of a Building a furlong off or to examine the pointing, &c. of the walls of upper stories, &c.
When shut up in its case, this little Glass is only 2 inches in length; when in use, about 3 inches: it has a single plano-convex Objectglass of an inch in diameter, and its Magnifying power is about 3 times.
* An Opera-Glass which only magnifies once, like the fashionable Grand Dandy Operas — of which the Eye-glass is as large as the Object-glass is of no use to a Nearsighted person;—who, to receive the same benefit which a common Eye does, will require rather more magnifying power:- A very short-sighted person will receive very little more assistance from an Opera-glass which magnifies Twice, than a person with a Common Eye will from one which magnifies Once.