of Edinburgh, Session 1872 - 73 . 103 
2. A flash of light, lasting the fraction of a second, produces a 
marked effect. 
3. A lighted match, held at a distance of 4 or 5 feet, is sufficient 
to produce an effect. 
4. The light of a small gas-flame, enclosed in a lantern, and 
caused to pass through a globular glass jar (12 inches in diameter), 
filled with a solution of ammoniacal sulphate of copper or bichro- 
mate of potash, has also produced a change in the amount of the 
electro-motive power. 
5. The action of light on the eye of the frog is as follows : 
— When a diffuse light is allowed to impinge on the eye of the 
frog, after it has arrived at a tolerably stable condition, the natural 
electro-motive power is in the first place increased, then diminished; 
during the continuance of light it is still slowly diminished to a 
point where it remains constant; and on the removal of light, there 
is a sudden increase of the electro-motive power nearly up to its 
original position. The alterations above referred to are variables, 
depending on the quality and intensity of the light employed, the 
position of the eyeball on the cushions, and modifications in the 
vitality of the tissues. 
6. Similar experiments made with the eye of warm-blooded 
animals, placed on the cushions as rapidly as possible after the 
death of the animal, and under the same conditions, have never 
given us an initial positive variation, as we have above detailed in 
the case of the frog, but always a negative variation. The after 
inductive effect on the withdrawal of light occurs in the same 
way. 
7. Many experiments have been made as to effect of light from 
different portions of the spectrum. This was accomplished by 
causing different portions of the spectrum of the oxy-hydrogen 
lime-light to impinge on the eye. All these observations tend to 
show that the greatest effect is produced by those parts of the 
spectrum that appear to consciousness to be the most luminous, 
namely, the yellow and the green. 
8. Similarly, experiments made with light of varying intensity 
show that the physical effects we have observed vary in such a 
manner as to correspond closely with the values that would result • 
if the well-known law of Fechner was approximately true. 
VOL, VIII. 
