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XIV. An Appendix to Mr. Ware's Paper on Vision. By Sir
Charles Blagden, F. R. S.
Read February 4, 1813.
MR. Ware states in his paper, that near sightedness comes on most frequently at an early age; that it is more common in the higher than in the lower ranks of life; and that particularly at the universities, and various colleges, a large proportion of the students make use of concave glasses. All this is exactly true, and to be accounted for by one single circumstance; namely, the habit of looking at near objects. Children born with
eyes which are capable of adjusting themselves to the most distant objects, gradually lose that, power soon after they begin to read and write; those who are most addicted to study become near sighted more rapidly; and, if no means are used to counteract the habit, their eyes at length lose irrecoverably the faculty of being brought to the adjustment for parallel rays. Of this I am myself an example, and as I recollect distinctly the progress, it may not be useless to record it here,
When I first learned to read, at the usual age of four or five years, I could see most distinctly, across a wide church, the contents of a table on which the Lord's Prayer, and the Belief, were painted in suitably large letters. In a few years, that is, about the ninth or tenth of my age, being much addicted to books, I could no longer read what was painted on this table; but
the degree of near sightedness was then so small, that I found a watch-glass, though as a meniscus it made the rays diverge very little, sufficient to enable me to read the table as before. In a year or two more, the watch-glass would no longer serve my purpose; but being dissuaded from the use of a common concave glass, as likely to injure my sight, I suffered the inconvenience of a small degree of myopy, till I was more than thirty years
That inconvenience, however, gradually though slowly increasing all the time, at length became so grievous, that at two or three and thirty, I determined to try a concave glass; and then found, that the numbers 2 and
3 to me in the relation so well described by Mr. WARE; that is, I could see distant objects tolerably well with the former number, but still more accurately with the latter. After contenting myself a little time with No. 2, I laid it wholly aside for No. 3; and, in the course of a few more years, came to No. 5, at which point my eye has now been stationary between fifteen and twenty years. An earlier use of concave glasses would probably have made me more near sighted, or would have brought on my present degree of myopy at an earlier period of life. If my friends had persuaded me to read and write with the book or paper always as far from my eye as I could see; or if I had occasionally intermitted study, and taken to field sports, or any employment which would have obliged me to look much at distant objects, it is very probable that I might not have been near sighted at all. Possibly the persons who become near sighted by having constantly to adjust their eyes to near objects, may not usually change to be long sighted by age.
On the subject of vision, I may be allowed to take this
opportunity of relating an experiment made many years ago, to decide how far the similarity of the images seen by each eye contributed to make them impress the mind as one. In the house where I then lived was a marble chimney-piece, the upper
horizontal block of which was fluted vertically; and the ridge between each concavity of the fluting was about as wide as the concavity itself. When I looked at this
of fluting at the distance of about nine inches, and directed the optic axes to it, I saw of course every ridge and concavity distinctly, and judged rightly of the distance. Adjusting the optic axes as to an object a little further off, I discerned the fluting confusedly and all double, the ridges interfering with the concavities; which was accompanied with the uneasy sensation of squinting. But on widening the direction of the optic axes still more, as to an object about eighteen inches distant; (namely, just so far that the duplication of the images should correspond successively; that is, so that the first ridge and concavity of the fiuting, as seen by one eye, should fall in with the second ridge and concavity, as seen by the other;) the fluting appeared as distinct and as single as at first; but it seemed to be about double the distance from the eye that it really was, and to be magnified in proportion; nor had I, in this case, any sensation of squinting. As the parts of the fluting, though in general much alike, were not exactly so every where in colour and minute circumstances, there appeared in some places a slight confusion from this dissimilarity of the images; but that trifling confusion had no manner of effect on the mind's judgment of the images, which looked as perfectly single, as when the futing was viewed with the optic axes so directed, that the ridges and concavities seen by
one eye corresponded with the same ridges and concavities as perceived by the other. No idea was suggested, but that of a range of futing larger and more distant than it was in fact. This experiment I frequently repeated, and always with the same effects.
XV. A Method of drawing extremely fine Wires. By William
Hyde Wollaston, M. D. Sec. R. S.
Read February 18, 1813.
It is recorded by Musschenbroek, that an artist of Augsburg drew a wire of gold so slender, that five hundred feet of it weighed only one grain; but the method by which this was effected is not mentioned, and indeed it has been doubted, whether it could really have been done. I shall however shew, that a wire of gold may, without much difficulty, be obtained finer than that spoken of by MUSSCHENBROEK, and that wires of platina may be drawn much more slender, with the utmost facility.
Those who draw silver wire in large quantities for lace and embroidery, sometimes begin with a rod that is about three inches in diameter, and ultimately obtain wires that are as small as sóo of an inch in thickness. If in any stage of this process a rod of silver wire be taken, and a hole be drilled through it longitudinally, having its diameter one-tenth part of that of the rod, and if a wire of pure gold be inserted, so as to fill the hole, it is evident that by continuing to draw the rod, the gold within it will be reduced in diameter exactly in the same proportion as the silver; so that if both be thus drawn out together till the diameter of the silver is só of an inch, then that of the gold will be only 5000; and of such wire five hundred and fifty feet would be requisite to weigh one grain.