582 ANIMAL PHYSIOLOGY. 
lasts the stimulus in these cases, and this is the explanation 
into which all the above-mentioned facts fit. When the fiery 
point passing before the eyes in the case of the fire-brand stimu- 
lates the same parts of the retina more frequently than is con- 
sistent with the time required for the previous impression to 
fade, there is, of necessity, a continuous sensation, which is in- 
terpreted by the mind as referable to one object. In like man- 
ner, in the case of a moving object seen by an electric flash, the 
duration of the latter is so brief that the object illuminated can 
not make any appreciable change of position while it lasts; a 
second flash would show an alteration, another part of the retina 
being stimulated, or the original impression having faded, etc. 
In the case of a top or (better seen) color-disk, painted into 
black and white sectors, it may be observed that with a faint 
light the different colors cease to appear distinct with a slower 
rotation than when a bright light is used. The variation is 
between about +5 and zy of a second, according to the intensity 
of the light used. Fusion is also readier with some colors than 
others. 
It is a remarkable fact that one can distinguish as readily 
between the quantity of light emanating from 10 and 11 can- 
dles as between 100 and 110. Weber’s law is a highly general- 
ized form of this statement applicable to all the senses. 
But with vision, as with all the senses, a lower and espe- 
cially an upper limit is soon reached, within which alone we 
can discriminate. It is not possible to distinguish between the 
difference in brightness of the central and the circumferential 
parts of the sun, though it is known that the actual difference 
is very great, while it is easy enough to recognize a marked 
difference in the light of a room when two candles are used in- 
stead of one. Within certain limits we can appreciate a differ- 
ence in illuminating power of about 74, of a given total. 
The Visual Angle—If two points be marked out with ink on 
a sheet of white paper, so close together that they can be just 
distinguished as two at the distance of 12 to 20 inches, then on 
removing them a little farther away they seem to merge into 
one. 
The principle involved may be stated thus: When the dis- 
tance between two points is such that they subtend a less visual 
angle than 60 seconds, they cease to be distinguished as two. 
Fig. 425 illustrates the visual angle. It will be noticed that a 
larger object at a greater distance subtends the same visual 
angle asa smaller one much nearer. The size of the retinal 
