is formed behind the retina; whilst in those who are short-sighted it is formed before. In nearly all such cases, this defect can be remedied by an artificial addition to the natural lenses of the eye, which tend to shorten its focal length in one case and to lengthen it in the other.

But there is a residual difficulty, even in well-constituted eyes, which remains to be explained. By what adjustment of its machinery does the same eye view both near and distant objects, without being sensible of confusion or obscurity; for it is utterly inconceivable that the same optical apparatus should form the images of near and distant objects at the same distance from the iris.

Much of the confusion, which would otherwise arise, is probably corrected by habit, and very possibly may form an important element in those delicate laws of aerial colouring and effect, by which the impression of distance as well as of form is produced. It is only when this confusion is very considerable, as in the case of very near or of very distant objects, that we become sensible of an effort within the eye itself, or in the parts connected with it, by which it is partially, if not entirely, corrected; in other words, when we look at very distant objects, the eye assumes a state which would make near objects appear confused, and conversely. It is inferred, therefore, that the eye possesses within itself, or in its adjuncts, the capacity of altering its focal length.

Leeuwenhoek, in the seventeenth century, had very accurately described and delineated the fibrous structure observable in the crystalline lens of various animals: and Dr. Henry Pemberton, of Oxford-the friend and commentator of Newton--at a subsequent period,

had conceived those fibres to be muscles, by whose action those changes in the form of the eye are produced which are required for the adequate explanation of the phenomena of near and distant vision. Young would appear to have been aware of Dr. Pemberton's hypothesis, though he had not studied his writings; and in dissecting the eye of an ox, when very recently slaughtered, he fancied that he had discovered, in the arrangement and attachment of those fibres, very satisfactory evidences of their muscularity. The Memoir in which his views were explained was read to the Royal Society on the 30th May, 1793, and was published in the Transactions for that year. It is written in very plain and lucid language, more like the style of a practised writer and anatomist than of a youth just entering upon his professional studies. The merit of this essay was considered to be sufficient to justify his election as a Fellow of the Society in the following year."

Circumstances gave to this first acknowledged publication of our author a greater degree of importance than it altogether deserved.

• The following is his certificate, copied from the Records of the Royal Society :

"Mr. Thomas Young, of Little Queen Street, Westminster, a gentleman conversant with various branches of literature and science, and author of a paper on Vision, published in the Philosophical Transactions, being desirous of becoming a Fellow of the Royal Society, we recommend him from our personal knowledge, as worthy of that honour, and likely to become a useful member of the Society.

It had no sooner appeared, than the great anatomist and physiologist, John Hunter, claimed the discovery, as his own, and after notifying his claim, he addressed an application to Sir Joseph Banks to be allowed to give the Croonian Lecture at the Royal Society for the following year, expressly upon the muscularity of the crystalline lens. A very small portion only of this lecture was completed at the time of his death, which took place in the following autumn," and the further prosecution of the investigation was resumed by his brother-in-law, Sir Everard Home, in the Croonian Lecture of the following year, in connexion with a series of apparently very accurate and welldesigned experiments made by himself and Mr. Ramsden, the eminent optician. The result of these experiments seemed very decisively to negative the existence of those changes of the form of the eye, in passing from near to distant vision-and conversely— which the hypothesis in question was designed to explain, and it was consequently very generally abandoned. Young himself, in the first instance, yielded to the force of this evidence, and in more than one publication afterwards announced that he no longer ventured to maintain his opinion in the face of such distinguished authorities."

See a short notice by Sir Everard Home, entitled 'Some facts relative to the late Mr. Hunter's preparation for the Croonian Lecture,' where the fragment which he had prepared is given.-Philosophical Transactions for 1794, p. 21. On the day of Hunter's death the following lines appear among Young's notes on the medical lecture he was attending, showing his deep sense of the loss which the science of physiology and medicine had thereby sustained :—

"Hei mihi! Quantum

Præsidium Ausonia et quantum tu perdis, Iule."

In his Göttingen Dissertation, to be noticed hereafter, published in 1796, he says:-"Sententia nuper de lentis crystallinæ usu in oculo ad

The reclamation by John Hunter of the proof of the correctness of this hypothesis, and the notice, by Sir Everard Home, of his proposed memoir on it--which was read to the Royal Society in the early part of the following year-gave rise to a rumour that Sir Charles Blagden-who was generally well acquainted with all that was passing in the philosophical world, and very much given to retailing it-had spoken of the subject in some detail, at a dinner party at the house of Sir Joshua Reynolds, on the 6th of November, 1791, where Dr. Lawrence, Dr. Walker King, Boswell, Dr. Brocklesby, and Young, were also present. The circulation of such a report and the plagiarism which it implied, was so injurious to Young's character, that he felt it necessary to address letters to all these persons, requesting them to say, whether the subject of vision and any recent researches connected with it, were mentioned on the occasion referred to. A very distinct denial was given by all the parties, as far as could be authorized by the vague recollections of a conversation reported to have taken place more than two years before, and Sir Charles Blagden-who would appear to have given currency to the report-assured Young that "he was by no means so clear as to be sure that he had told him Hunter's opinion." The imputation was distinctly withdrawn by him, and was speedily forgotten; no one, in fact, who was acquainted with the scrupulous regard for truth for which Dr. Young was always distinguished, could ever have given a moment's credit

to it.

diversas rerum videndarum distantias accommodando proposita, neque nova neque vera videtur." A similar acknowledgment is also made at the end of his Memoir in the Philosophical Transactions for 1801, read in November, 1800, entitled Outlines of Experiments and Inquiries respecting Sound and Light.'

We shall anticipate somewhat on the current of our narrative, to refer to Dr. Young's subsequent views on this controverted subject. We have before noticed his candid abandonment of his first opinion: he resumed it, however, afterwards, and embodied his reasons for doing so in a very able and elaborate memoir, "On the Mechanism of the Eye," which was read to the Royal Society on the 27th November, 1800, and published in their Transactions for the following year.

The experiments of Sir Everard Home and Mr. Ramsden had seemed to prove, that, in the adjustment of the eye to different distances, it is the curvature of the cornea and the length of its axis-and not of the crystalline lens-which is changed; and further that the eye of a man which had been couched, or deprived of its crystalline lens, was perfectly susceptible of this adjustment, which therefore could not be dependent upon it. It was the assertion of this startling and apparently unanswerable fact, upon such high authority, which at first induced Dr. Young to revoke his original opinion; but it was a very serious objection to its adoption, that it would require an amount of change in the cornea, and an extension of the sclerotica, which there is no adequate anatomical provision to produce, or safely to apply. Dr. Young, by means of an improved form of Dr. Porterfield's optometer, an instrument admirably adapted to measure the focal length of the eye, and by numerous experiments, both on eyes which had and which had not, been couched, was enabled to negative any sufficient change in the curvature of the cornea in all cases, and further, decisively to show that couched eyes had no power of adjustment to near and distant objects. He thus altogether reversed the conclusions of his predecessors in this inquiry, and