THE SENSES. 725 



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 in- 

 crease in the effect is not directly proportional to the increase in the 

 excitation, but, according to Fechner's 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 minimal increment of excitation. 



This law, which is true only within certain limits, may be best 

 understood 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 

 sensation 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 addi- 

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

 stimulus 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. . 



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 ihefundus 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 retinal pigment, as mentioned above ; but far more to the fact that 

 every such ray is reflected straight 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 is surmounted by the use of the ophthalmoscope. 



The ophthalmoscope, brought into use by Helmholtz, consists in its simplest 

 form of a, a slightly concave mirror of metal or silvered glass perforated in 

 the centre, and fixed into a handle ; and b, a biconvex lens of about 2^-8 inches 

 focal length. Two methods of examining the eye with this instrument are in 

 common use the direct and the indirect: both methods of investigation should 

 be employed. A normal eye should be examined ; a drop of a solution of atro- 

 pia (two grains to the ounce) or of homatropia hydrobromate, should be in- 

 stilled about twenty minutes before the examination is commenced ; the ciliary 

 muscle is thereby paralyzed, the power of accommodation is abolished, and the 



