Fig. [2]. represents the eye at the time when spectacles are required, the cornea, or the chrystalline, or both, having lost a part of their natural convexity, consequent upon age or constitutional weakness. An object placed at the same distance from the eye as in the perfect eye, (Fig. [1]), has the focus carried beyond the retina. A convex lens applied to the eye compensates for this loss of capacity, and, converging the rays, corrects the focal distance, and the image is now imprinted naturally on the retina. The reverse of this takes place in the case of the short-sighted; the humours being more convex than in the perfect eye, the rays converge in a focus before they reach the retina. A concave lens carries the focus further on, and, by its aid, an object will be depicted perfectly on the retina.
No. 4 represents a plano convex lens—5, a double convex—6, a plano concave—7, a double concave—8, a periscopic or meniscus lens.
What grandeur and sublimity of contrivance is here blended with simplicity of action and power of expression! How indispensable to a full appreciation of the bounties of nature and the beauties of art by which we are surrounded! To be deficient of this heavenly gift is truly to have “wisdom at one entrance quite shut out.”
Surely every consideration should influence us to treat this invaluable faculty with judicious care, instead of allowing it to be tortured and trifled with by the ignorant and unprincipled.
Having surveyed the general construction, and glanced at the inimitable mechanism of those windows of the soul, we shall be better prepared to understand the reasoning, and to comprehend the principles upon which the science of optics is based. Those of my readers who have leisure to pursue this subject, will find new light, and more convincing illustrations attend their enquiry at every step. It is a subject abounding in beauty and interest, introducing us to new regions of sublimity and grandeur, where the contemplative mind will assuredly find “ample scope and verge enough” to gratify its most exalted anticipations. We have seen the admirable, yet at the same time delicate contrivances by which the functions of the eyes are performed. It cannot fail to have occurred to us, that a machine so beautiful and complete is liable to derangement and improper treatment by the wayward and the ignorant. If it is desirable that a person possessed of a well-constructed watch should understand its general action, and know what treatment it should have to keep it in sound and underanged order, still more essential is it that every individual should possess a clear and familiar knowledge of the nature of vision, and understand the requirements of the eyes. I cannot forbear indulging a sanguine hope that the circulation of this little manual will open the eyes of its readers to the simple facts of the case, and animate them to think and judge for themselves, instead of giving a ready ear to the marvellous and ridiculous tales, which, though, like the sailor’s tough yarn, so often told that the cheat believes them himself, are nevertheless utterly devoid of truth.
FOOTNOTES:
[1] In the month of August, 1838, the solicitor to the Duke of Wellington waited upon Sir Frederick Roe, at Bow-street, in reference to the use which had been made of his grace’s name by “The London Equitable Loan Company.” A gentleman, who was desirous of purchasing shares in the company, seeing the Duke of Wellington mentioned as patron, and that the account of the company was kept with the Bank of England, wrote to the Duke to ascertain if it were true that he was connected with the concern? The duke never had consented to become a patron, but, on the contrary, when asked to do so, answered that he was so situated that he could not comply with their request. On receiving the communication the duke wrote to Sir F. Roe, to see what could be done to guard the public against any transaction which might be entered into in consequence of the use made of his name; and in answer to a letter written by him to the Bank of England, he was informed “that no such company was known there, nor had they any account of the London Equitable Loan Company.” The publicity given to this manœuvre has no doubt saved many a family from enthralment and destitution.
CHAPTER II.
“The subject is of universal importance, since every man, woman, and child in the empire will probably require optical assistance.”—Abbott.
The eyes, when in a sound and healthy state, instinctively adjust themselves at a distance of twelve inches from a book or paper, when they are observing the same. This distance is found to be most natural and agreeable; for when we extend it to sixteen, twenty, or thirty inches, the chrystalline lens is stimulated to keep a distinct and clear perception, until, as the distance increases, the object becomes less and less perceptible. When we are compelled to extend this natural distance, experience difficulty in reading small characters, or find it necessary to get more light on what we are observing, we may safely conclude that artificial assistance is needed, and that, judiciously applied, the tendency to decay will be mildly arrested.