Vision in health and disease : the value of glasses for its restoration, and the mischief caused by their abuse : being the substance of lectures delivered at the Central London Ophthalmic Hospital / by Alfred Smee.

  • Smee, Alfred, 1818-1877.
Date:
1847
Catalogue details

Licence: Public Domain Mark

Credit: Vision in health and disease : the value of glasses for its restoration, and the mischief caused by their abuse : being the substance of lectures delivered at the Central London Ophthalmic Hospital / by Alfred Smee. 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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    11- Fig. 12. spot; and, perhaps, it is for this reason, as well as for the estimate of angles, that perfect vision is confined to a mere spot, or rather to the small angle which I before mentioned to you. A pretty experiment demonstrates that single vision de- pends uj)on objects being seen in their true place; for if we regard a star or gas-light at a distance (a, fig. 12) with both eyes directed towards it, we see but one star. If however, we suddenly look at b, an object but two or three inches off, the rays of light can still fall upon the retina, but in such a manner that the direction vertical to a tangent causes one star to appear at a1, and the second at a2. It is a marvellous sight to observe one star divide into two and shoot away into their false positions, if the experiment be well performed. (27.) We judge of the relation of objects to each other, and whether they be horizontal or vertical, by the position taken by ourselves, as well as by the position of known surrounding objects. (28.) There is a limit to vision: we cannot see objects which are below a certain size, or, rather, which do not subtend a certain angle; thus, the largest object may be rendered invisible by removing it to a sufficient dis- tance, and conversely the smallest may be rendered visible by increasing the visual angle. The inverted telescope will render objects invisible by diminishing the angle; and the microscope renders objects, which were previously invisible, apparent by increasing the angle. Doubtless, persons differ in the extent to which they can perceive objects: many cannot see below 40; but I find that I can see the round head of a pin one-twentieth of an inch in diameter, at fifty feet distance, which will only afford an angle of 17, and I have even been able to see a fine platinum wire under cir- cumstances which did not afford an angle of more than 4. (29.) When we regard an object, we are enabled to receive a sustained impression, that is, we may look at it steadfastly, and see it for some time. This sustained impression only exists in the centre of the eye, and for strong contrasts of colour: at the sides of the eye it is intermittent it becomes invisible, then visible again, a property which Sir D. Brewster first indicated. Even in the centre of the eye, after a long period, it will at last fail or fade away. (30.) Inasmuch as I have avoided the details of mathematical disqui- sition, so shall I also avoid the minuti® of optical science further than to demonstrate briefly to you the manner in which images are formed by convex lenses. It is a property of these lenses to bring parallel rays which fall upon them to a focus, or point, (f fig. 13 ; a, fig. 14,) and the dis- tance between this point and the lens is called the focal distance. Divergent rays will cause the focal distance (fig. 16) to be lengthened according to the amount of divergence ; and convergent rays will cause the focal distance