. Kirkes' handbook of physiology . e stimulus, but is proportional to the intensity of thestimulus already acting, Webers law. This law, which is true only within certain limits, may be best under-stood by an example. When the retina has been stimulated by the light ofone candle, the light of two candles will produce a difference in sensationwhich can be easily and distinctly felt. If, however, the first stimulus is thatof an electric arc-light, the addition of the light of a candle will make no dif-ference in the sensation. So, generally, for an additional stimulus to be felt,it may be propor
. Kirkes' handbook of physiology . e stimulus, but is proportional to the intensity of thestimulus already acting, Webers law. This law, which is true only within certain limits, may be best under-stood by an example. When the retina has been stimulated by the light ofone candle, the light of two candles will produce a difference in sensationwhich can be easily and distinctly felt. If, however, the first stimulus is thatof an electric arc-light, the addition of the light of a candle will make no dif-ference in the sensation. So, generally, for an additional stimulus to be felt,it may be proportionately small if the original stimulus is small, and mustbe greater if the original stimulus is great. The stimulus increases as thenumbers expressing its strength, while the sensation increases as thelogarithms. Every one is familiar with the fact that it is quite impossible to seethe fundus or back of another persons eye by simply looking into it. Theinterior of the eye forms a perfectly black background to the pupil. The same. Fig. 468.—Diagram to Illustrate the Action of the Ophthalmoscope when a Plane ConcaveGlass is Used, c. Observers eye. The light reflected from any point, d, on retina of o, wouldnaturally be focussed at c; if the lens b is. used it would be focussed at i, in other words, at backof c. The image would be enlarged, as though of g, and would be inverted. (After McGregor Rob-ertson.) remark applies to an ordinary photographic camera, and may be illustratedby the difficulty we experience in seeing into a room from the street throughthe window, unless the room be lighted from within. In the case of theeye this fact is partly due to the feebleness of the light reflected from theretina, most of it being absorbed by the retinal pigment. But the difficultyis due more to the fact that every such ray is reflected back to the source oflight and cannot be seen by the unaided eye without intercepting the in-cident light as well as the reflected rays from the retin
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