Copy 1

A treatise on optics / by David Brewster.

  • David Brewster
Date:
1831
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on optics / by David Brewster. Source: Wellcome Collection.

    322/418 (page 296)
    that axis; so that we are not entitled to ascribe the distinct- ness of vision in the axis to the circumstance of the image being formed on the central hole of the retina, where there is no nervous matter; for if this were the case, there would be a precise boundary between distinct and indis-= tinct vision, or the retina would be found to grow thicker and thicker as it receded from the central hole, which is not the case. In making some experiments on the indistinctness of vision at a distance from the axis of the eye, I was led . to observe a very remarkable peculiarity of oblique vision. If we shut one eye, and direct the other to any fixed point, such as the head of a2 pin, we shall see indistinctly all other objects within the sphere of vision. Let one of these objects thus seen indistinctly be a strip of white paper, or a pen lying upon a green cloth. Then, after a short time, the strip of paper, or the pen, will disappear altogether, as if it were entirely removed, the impression of the green cloth upon the surrounding parts of the eye extending itself over the part of the retina which the image of the pen occupied. In a short time the vanished image will re-appear, and again vanish. When both eyes are open, the very same effect takes place, but not so rea- dily as with one eye. If the object seen indistinctly is & black stripe on a white ground, it will vanish in a similar manner. When the object seen obliquely is luminous, such as a candle, it will never vanish entirely, unless its light is much weakened by being placed at a great dis- tance, but it swells and contracts, and is encircled with a nebulous halo; so that the luminous impressions must ex= tend themselves to adjacent parts of the retina which are not influenced by the light itself. If, when two candles are placed at the distance of about eight or ten feet from the eye, and about a foot from each other, we view the one directly and the other indirectly, the. indirect image will swell, as we have already men- tioned, and will be surrounded with a bright ring of yellow light, while the bright light within the ring will have a pale blue colour. If the candles are viewed through a