On sight, and its relation to inverted images / by Wm. Munro.

  • Munro, William
Date:
[1858]
Catalogue details

Licence: Public Domain Mark

Credit: On sight, and its relation to inverted images / by Wm. Munro. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    oil a limited spot, yet when tliey fall on tins delicate curtain, the object 18 inverted, but the anterior or inner layer of nerve cells soon bring thoni beyond the perpendicular again, so that be fore they arrive at tlic fibres which lie between the two rows of cells, the image is really erect. Such a conclusion would not be doubted by any one, if there were really only one of these cells, and that the inverted im- age really fell on its anterior surface, but because there are so many of tliese, many would prefer to stick to the old opinion, not discern- ing that the muUitade of these cells really make the image more certainly perfect, at whatever angle any outward ray may reach the eye, and tliat it mattered not, whether or not there was a curtain to receive it. Were it not so, let us reason the matter and see what the consequence would be, if tlie image was erect, when it fell on this curtain, and we will find that after it had passed through these anterior cells it would, instead of giving an erect image to the brain give an inverted one, as we must recollect, that though these small bodies measure after death scarcely Tro^th part of an inch in diameter, and probably, though full not much larger during life, yet though they were much smaller, they are actually translucent, double con- vex, and of a greater density ,than the vitreous humour. Under such circumstances, having minute radi|^they are capable not only of acting as lenses, but as very powerful ones, in bringing the inverted image riipidly into an erect position, even before arriving at the fibres which seperate these anterior from the posterior cells. It may not moreover be too much to infer that the fibres which scpar. ate the two rows of cells, if they do not convey the sensation of sight through their own instrumenta-lity, act as in secreting glands, the part of a basement membrane, and allow the peculiar requisite for sight to pass through lo be absorbed by the posterior la^er, and by them to be carried to the brain. The common tire wheel of the school boy, seems also a physio- logical proof, that these small cells, do individually carry the sensation of images to the brain, for in thi,s though it actually appears to the eye to be a continuous sheet of flame, is nothing but a red point whirled ra])idly round in the air, from which rays pass as quickly firstto one of these cells and then to another,thus keeping up the allusion. Other instances of the same kind are found in those parti-coloured deceptions seen in some drawing rooms, as also in our common kaleidesc'pes. When one looks at the eye of the fly, we see there a number of little conical projections, which though on the surface of that insect, somewhat resembles these small cells, and which must undoubtingly answer the same purpose, but be in them a complete optical appara- tus. Yet still though there be so many we have no reason to think that they convey to the sensorium a number of images but a single one. Such a wonderful system of double convex lenses formed by the anterior portion of the optic nerve, not only would individually correct the inverted image, but if one set failed, others would supply the deficiency, as in all probability they do in the fly. It is for the same reason that in a dim light the pupils expand to allow more rays to fiill on the retina, making a larger image t » be formed on its deli- cate screen, and a larger number of these small bodies to act upon it,