THE SENSES. 217 



intensity of the sensation is not directly proportional to the intensity of 

 the luminosity of the object. It is necessary for light to have a certain 

 intensity before it can excite the retina, but it is impossible to fix an 

 arbitrary limit to the power of excitability. As in other sensations, so 

 also in visual sensations, a stimulus may be too feeble to produce a sen- 

 sation. If it be increased in amount sufficiently it begins to produce an 

 effect which is increased on the increase of the stimulation; this increase 

 in the effect is not directly proportional to the increase in the excitation, 

 but, according to Feclmers law, "as the logarithm of the stimulus," i.e., 

 in each sensation, there is a constant ratio between the increase in the 

 stimulus and the increase in the sensation, this constant ratio for each 

 sensation expresses the least perceptible increase in the sensation or min- 

 imal increment of excitation. 



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 

 of one candle, the light of two candles will produce a difference in sensa- 

 tion which can be distinctly felt. If, however, the first stimulus had 

 been that of an electric light, the addition of the light of a candle would 

 make no difference in the sensation. So, generally, for an additional 

 stimulus to be felt, it may be proportionately small if the original stim- 

 ulus have been small, and must be greater if the original stimulus have 

 been great. The stimulus increases as the ordinary numbers, while the 

 sensation increases as the logarithm. 



The Ophthalmoscope. Part of the light which enters the eye is 

 absorbed, and produces some change in the retina, of which we shall treat 

 further on; the rest is reflected. 



Every one is perfectly familiar with the fact, that it is quite impos- 

 sible to see the fundus or back of another person's eye by simply looking 

 into it. The interior of the eye forms a perfectly black background to 

 the pupil. The same remark applies to an ordinary photographic camera, 

 and may be illustrated by the difficulty we experience in seeing into a 

 room from the street through the window, unless the room be lighted 

 within. In the case of the eye this fact is partly due to the feebleness of 

 the light reflected from the retina, most of it being absorbed by the cho- 

 roid, as mentioned above; but far more to the fact that every such ray is 

 reflected straight back to the source of light (e.g., candle), and cannot, 

 therefore, be seen by the unaided eye without intercepting the incident 

 light from the candle, as well as the reflected rays from the retina. This 

 difficulty has been surmounted by the ingenious device of Helmholtz, 

 now so extensively used, termed the ophthalmoscope. As at present used, 

 it consists of a small slightly concave mirror, by which light is reflected 

 from a candle into the eye. The observer looks through a hole in the 

 mirror, and can thus explore the illuminated fundus; the entrance of the 

 optic nerve and the retinal vessels being plainly visible. 



