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Distinct And Indistinct Vision

Distinct And Indistinct Vision image
Parent Issue
Day
6
Month
June
Year
1894
Copyright
Public Domain
OCR Text

The mechaiusm of distinct and indistijict visión has been understood only since 187G. The sensative parts of the retina are little rods and cones forming a layer by themselves. In 1876 BoJl diecovered that in frogs kept in the dark tlie rods 'of the retina were colored dark purple ; but on exposure to ligh.t the color faded, beoomimg first yellow and then white. Sin&e that time, physiologists have been carefully investigating visual purple and visual yellow. Just outoutsddO th.e layer oí rods and cones are the dark oells which render the greatest part of the interior of the eye aJimost black, üi the dark, these &ells eend little filaments between the rods and discharge a liquid which colors the rode alone. When the rods are thus oolored, t!ie eye is extremely eensative, so tlxat a bright light is dazzling and painful and obseuree distiinct visión. TJüe is the reason why we cam not see distinctly when we come suddenly from the dark into a full light. ín a few seconde, howerer, the color is bleaclied to a yellow and the difficulty passes away. "Wheai, 011 the other liand, we pass from n bright light hito the dark, the rethia lias loet it seusibility írom disappearance of tlie vúsual purple, and we can not see at all until tlie purple is reproduced, qís it is in tlie absence of light. Tlie dit'fíereiice is not due to dialation of the pupil in the dark amd tontractiou under fche influence of liglit, as is popularly supposed, for a pe-rson does not see better in the dark when the pupil has been fully diilated ty belladonna. In the little orea oí distinct visión there is inever any visual purple. Tliis area we always use with sufficient light for miaiute details of objeets, making tlien the great-est use oí the mechanism of accommodation. The area onitside of thie is used for indistinct visión, and as tlie color is then yellow instead of purple, it is only moderately sensative. To express the oomditions in a few words, the minute area ior distinct visión is used by day, and the area for indistinet visión, with lts visual purnle. is used

Article

Subjects
Old News
Ann Arbor Courier