
PHYSIOLOGICAL ACTION OF LIGHT. 157 
exhaustion of the retina and the action of light during different times. All 
such questions are left for examination in a subsequent paper on the effects of 
fatigue. Similar results to the above have been found for the compound eye. 
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, &¢., and leave 
only perception of difference of intensity? In other words, are these changes 
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 theo- 
retically and experimentally by the justly renowned German physiologist 
FECHNER, and more recently extended and verified by DaLsaur of the Belgian 
Academy. FEcHNER’s law as applied to light may be stated as follows :—If we 
call I and S respectively the luminous and sensational intensities, and a and & 
constant quantities, then the following equation is found to express the relation 
of the above quantities, viz. :— 
S=atlogI +4 : , ; OD) 
From this it follows, if we take any two new values, 8, and L,, we get the 
following relation— 
Si or= @ los (x) : — (2). 
That is, the difference of our sensations is directly proportional to the logarithm 
of the quotient of the luminous intensities. The formula (1) is, however, defec- 
tive when considered with regard to small values of I,and gives an imaginary 
quantity when it becomes zero, viz., infinity. In order to remedy this defect, 
Datse@ur has introduced a constant quantity along with the value of the lumi- 
nous intensity, that may be regarded as representative of the normal condition 
of the retina in darkness, or the proper light of the eye. Not that there is a 
certain amount of light when the eye is kept in the dark, only that the receiving 
organ has a natural susceptibility that must be included in the equation. 
The expression (1) becomes when we insert this constant quantity, C, 

S=alog(C+1I)+é . ; (3). 
And as § must be nothing when I is zero, we have— 
C+1 
S = @ log ( a ) : ; (4). 
- This formula has been verified experimentally by Datsc@ur, using the method 
of successive contrasts introduced by PraTeav, and from it he has succeeded 
in deducing the value of the unknown constant C. The value of this constant is 
_ found in his experiments to vary at different times between the values 0:1 and 
VOL. XXVII. PART I. De 
