him in a short time to wear Spectacles. As it proceeds he is under the necessity of using others of a higher power. But, instead of supposing that his Sight is gradually becoming worse, from a natural process, he attributes the increase of the defect in it, to his too early and frequent use of Glasses. Upon the whole, I should draw this inference from what has been said, that no person whose Sight begins to grow Long, ought to be in the least prevented from enjoying the immediate advantage which Spectacles will afford him, by the fear that they will ultimately injure his Eyes.” — DR. WELLS on Vision. CHAPTER VI. OF PRESERVERS. By the help of Convex Glasses* of 36 or 30 Inches focus, if your Eyes are in the state above mentioned, You will be enabled to read with the same ease, and at the same distance, * Of Convexes, i. e. Glasses for assisting those Eyes which are too Long Sighted— or what is commonly called that you did before your Sight was altered by the inevitable decrees of Old Time — such Glasses, will make things appear clear, and distinct as they did before your Eyes were impaired; and if judiciously chosen, lessen the labour of the Eyes and enable them to do their work with more ease, and therefore, I suppose, do, in a certain degree, preserve the Sight. This Title of PRESERVERS, which some sagacious name-giver gave to Spectacles of 36 Inches focus or the First Sight, is an admirable appellation to attract the attention of people but is equally applicable to all the following gradations of Glasses; for the term is generally misunderstood, - people seem to suppose, that Spectacles of 36 Inches focus, have the magic power of arresting the progress of that failing of the faculty of Sight, an Old Sight, are named from their focal length; the highest number, No. 36, magnifies least, and is called the 1st Sight: See Figure 5 in the Plate facing the Title. Of Concaves, i. e. Glasses for a Short Sight, that which is the least concave, and gives the least assistance to the Eye, is called No. 1. See Figure 7 in the Plate fronting the Title. which is one of the natural and unavoidable consequences of Age. When once the Sight begins to fail, it continues to decay, till in extreme Age, our Eyes, like our other Senses, become of almost as little use to us, as they were in our Infancy. It is very common for Persons after they have worn Spectacles of 36 Inches focus for a Year or two, to complain that they think that "their Glasses cannot be of the right focus for their Eyes, for when they do not wear them, they certainly cannot see so well without them as they did before they used them, therefore,they certainly cannot be Preservers.” After the process of deterioration has been. proceeding for 12 months-it would be wonderful if it had not made some perceptible progress! It is as ridiculous, to expect that it is in the power of an Optical Instrument to entirely prevent the Eye undergoing that invariably certain, although almost imperceptible change which accompanies the advance of Age, as it would be to suppose that Art can prevent the failing of any of our other faculties. I make these remarks, because, I know that the prepossessing term has induced some ex cellent Artists,* who were naturally extremely anxious about their Eyes-(their Eyes are their Estate, the mainspring of their Fame and of their Fortune) to wear Spectacles before they wanted them. MEM. The premature use of Spectacles, is as pernicious to the Sight, as Physic to the Stomach of a Man in Health, and as absurd, and as uncomfortable, as it would be to put on a Fur pelisse at Midsummer, as a Preserver against your feeling the Cold of Christmas. * See Chapter xiii. FOCAL LENGTH OF CONVEX GLASSES, &c. 33 CHAPTER VII. TABLE Of the Focal Length of the Convex or Magnifying Glasses commonly required at Various Ages. The above Scale was given to me by an eminent Optician, as the average results of upwards of 50 Years' very extensive Experi * These three last deep Lenses are very rarely required except for Couched Eyes. "One focus is seldom sufficient D |