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of Edinburgh , Session 1872-73. 
portion, is only reduced to one-third. Repeated experiments made 
with the eye in different positions have conclusively shown that a 
quantity of light one hundred times in excess of another quantity 
only modifies the electro-motive force to the extent of increasing it 
three times as much, certainly not more. 
9. It was apparent to us that these experiments would ultimately 
bear upon the theory of sense-perception as connected with vision. 
It is now generally admitted that no image, as such, of an external 
object, is conveyed to the sensorium, but that in reality the brain 
receives certain impressions of alterations taking place in the 
receiving organ. The natural query then arises — are the physical 
effects we have described and measured really comparable in any 
way with our sensational differences in light perception when we 
eliminate all mental processes of association, &c., and leave only 
perception of difference of intensity? In other words, are these 
changes the representative of what is conveyed to the sensorium ? 
It would appear, at first sight, that this problem is altogether beyond 
experimental inquiry. There is, however, a way of arriving at very 
accurate measures of the variation of our sensational differences in 
the case of light, and this has been developed theoretically and 
experimentally by the justly renowned physiologist Fechner. Stat- 
ing the law of Fechner* generally, we may say, the difference of 
our sensations is proportional to the logarithm of the quotient of 
the respective luminous intensities. A recent series of experiments 
by Dalboeuf f has entirely confirmed the truth of this law. If, 
therefore, the observed differences in electro-motive power, regis- 
tered under conditions of varying luminous intensity, agree with 
this law of Fechner, regulating our sensational impressions, then 
there can be little doubt these variations are the cause of, and are 
comparable to, our perception of sensational differences. Now, we 
have stated above, that with a quantity of light one hundred times 
in excess of another quantity, the electro-motive force only becomes 
three times greater. According to Fechner’s law, we may say the 
difference of our sensations, with that variation in the amount of 
luminous intensity, would be represented by 2, the logarithm of 
100. Our experimental results being as 3 to 1 3 the difference is 
* Fechner, Elemente der Psychophysik. Helmholtz, Optique Physiologique. 
t Recent Memoir to Belgian Academy. 
