450 
WILLIAM FERREL. 
more reasoned that if this retardation is inappreciable, then 
this must be due to the fact that it is counterbalanced by an 
acceleration of the rotatory speed due to the contraction of 
the earth as it slowly cools. 
(2.) Ten years later Ferrel was able to show that the ac¬ 
celeration of the moon’s mean motion, which is about 11 
seconds per century, is probably due to the outstanding ex¬ 
cess of tidal retardation just mentioned, whereby our day is 
shortening, thus producing an apparent acceleration of the 
moon’s motion. 
(3.) In a short paper of 1854, “ On the Internal Fluidity 
of the Earth,” Ferrel showed that we must consider the 
pressure quite as much as the temperature in discussing the 
question of the fluidity of great masses of solid matter. 
(4.) In a paper on “Vision,” in 1855, Ferrel showed, both 
by theory and experiment, that Brewster’s and other theories 
as to the nature of the process by which the mind recog¬ 
nizes the direction and dimensions or shape of a given 
object are at fault, and that the actual phenomena of stere¬ 
oscopic or binocular vision are only explicable on the 
principle that the mind bases its judgment as to the direc¬ 
tion of an object by a consideration or consciousness of the 
relation between the location of the particular cell of the 
optic nerve upon which the image of that object is formed 
and the location of the optical center of the crystalline lens; 
furthermore, that the mind bases its judgment as to the dis¬ 
tance of an object by a mental comparison of the location of 
the two coordinated nerve cells, one in each eye, on which 
the respective images of the object are formed. Ferrel es¬ 
tablished his explanation by many experiments, and his 
idea finds an especially severe test of its correctness by con¬ 
sidering, first, the cases in which the mind can be deceived 
as to the distance of an object as seen by persons of normal 
vision, and, second, the cases of strabismus where the mind 
is not deceived, although the eyesight is abnormal. 
(5.) In 1855, in a short paper on the “Variable Star 
Algol,” Ferrel showed that the steady shortening of its 
