[3] Pebbles, again, are proved by grinding the edges briskly on a moderately smooth file, or porous stone; they will resist the action of these, and emit sparkles of light as the velocity of the friction is increased, while glasses, on the contrary, yield, and are ground without difficulty. CHAPTER III. “Every day’s observation shews how far some men may be carried from the most evident and obvious truths, to support a new hypothesis, which has no foundation either in nature or reason; but truth will ever control and prevail over error, though supported with all the powers of rhetoric and novelty.”—B. MARTIN. Where distance, or any other cause, prevents our having access to persons who are competent to suit us properly with spectacles, great care should be exercised to choose those which cause the print of a newspaper, or the ordinary types of a book, to appear of their natural and proper size, while they are viewed at the distance most agreeable for reading, viz., twelve inches from the eye. It is evidently better to “try spectacles” on such ordinary printing as they will be generally exercised upon, than to test them by the very small print with which persons frequently provide themselves. To decipher this, a greater power is demanded than is requisite for usual reading, writing, and needlework, which are the purposes for which we require them, and not for microscopic observation, nor for distant views. The eyes in which no malformation or disease exists, but which simply partake of constitutional decay, or, from too continued application to sedentary and studious pursuits, are beginning to feel a want of assistance, should have spectacles of sixty-inch focus, which is an exceedingly slight magnifying power; and if these are found to be insufficient to afford an agreeable and natural perception (not an enlarged or magnified image of the letters of a book, &c., held in the hand at the distance of twelve or fourteen inches from the eye), then apply those of the next power, viz., forty-eight inches’ focus. If these again are unequal to supply the loss of power or incapacity of the eye to converge the light to a point at the instant it reaches the retina, then lenses of thirty-six inches focus are to be had recourse to; and when these fail to afford agreeable vision, thirty, twenty-eight, twenty-four, twenty, must be progressively adopted, thus gradually descending the scale until the eyes receive such compensation for their progressive decay and loss of power, as spectacles carefully suited to the sight are capable so effectually to supply.