194

INVERSION OF THE IMAGE.

that, if the image were inverted on the retina, the object would appear to the eye to be upsidedown, whereas we see men standing on their feet, and other objects in their true position. Some persons think the difficulty admits of easy explanation; but I never met with one that could be rendered quite satisfactory to you. It is, therefore, much better to leave this question for the present, and employ our thoughts on those particulars which you can distinctly understand.

We have now, I believe, considered the chief points of resemblance between the eye and the camera-obscura. We have seen that, in both, the image is formed in a dark chamber, by reflection of rays transmitted through a double convex lens. Before we examine the resemblance of the eye to a telescope, I wish to call your attention to one circumstance attending the formation of the image on the retina, which is truly wonderful. Perhaps you have seen and admired certain feats of minute penmanship, such as the Lord's Prayer written in a circle of the

MINUTENESS OF THE PICTURE.

195

size of a sixpence; but what would you say to a painter, who should delineate, in a landscape of that size, all the objects that can be seen from the top of Bow Hill, with the whole sweep of the coast, from the hills of the Isle of Wight to the cliffs beyond Brighton? Yet all this multitude of objects, this wide extent of prospect, is impressed on the retina of the eye that beholds it, in correct position, form, and colour! We might also, in some situations, watch a stagecoach travelling along for half-an-hour, and be distinctly sensible of its progress, see it go up one hill and down another; yet, in all that time, the tiny image would not pass over more than one-twelfth of an inch in the minute picture painted on the retina.* Who need seek to gratify their taste for the marvellous, by reading tales of giants, fairies, or enchanters, when the wonders of our own frame are more astonishing than them all ?

I will now endeavour to give you some account of the humours of the eye,

which may be

*Paley, Nat. Theol. 21.

196

MR. HALL'S DISCOVERY.

compared to the lenses of the telescope, in figure, position, and power over the rays of light. In a former Lecture, when speaking of the coloured fringes which were so great an imperfection in the early telescopes, I said that one ingenious person had fallen into the right train of thought respecting it, which led him to make a great discovery. That person was a gentleman of Essex; his name was Hall: and about the year 1729, while he was considering the difficulty, it came into his mind to inquire how this matter is managed in the eye:-then, reflecting that the humours of the eye are composed of substances which possess different powers of refraction, he thought that, if he could find substances having such powers as he supposed the humours of the eye might possess, he should be able to construct an objectglass which would be free from the coloured fringes. He accordingly made many experiments, and at length succeeded in finding the properties he desired, in two different kinds of glass. About four years after he first turned

his attention to the subject, he completed several object-glasses, which showed the object in its own tints, without confusing it by the fringe of prismatic colours. They were, in fact, like Dollond's achromatic glasses, though Mr. Hall did not give them that name.

When ingenious men have made a discovery which is likely, from its great utility, to become a source of wealth, they frequently take out a patent; that is, they obtain a writing from lawful authority, limiting the right of making the new article to themselves, for a certain specified time; and if any other persons infringe upon this right, they are liable to be punished for it in a court of justice. A patent was taken out, or requested, for making achromatic telescopes, whether by Dollond, I am not sure; but the right was disputed, and the matter brought to trial before Lord Mansfield, at Westminster Hall. From the evidence, it was clear that Mr. Hall was the inventor; but the judge remarked, that "It was not the person who locked up his invention in his desk who ought to profit by a

198

LORD MANSFIELD'S OPINION.

patent for such an invention; but he who brought it forth for the benefit of the public." Whether the opinion of Lord Mansfield influenced the decision of the jury, I cannot tell you; but Dollond is generally regarded as the inventor of achromatic telescopes; and they continued to be made and sold by himself and his sons for many years. Mr. Hall was a gentleman of property, who desired no pecuniary benefit from the discovery; therefore it seems most probable that his claim to the honour was brought forward by some instrument-maker, who wished to prevent other persons from securing a profitable manufacture to themselves.

The question of the patent is of no consequence to us; but I thought the facts would interest you, as being connected with the difficulty which led Mr. Hall to reflect on the structure of the eye, and to contrive a telescope made on the same principle.

The eye, as you may perceive from the section, is divided by the crystalline lens into two cham

*Hutton, ib. i. 26. Paley's Nat. Theol. 15. Arnott, ii. 296.