126 | PHYSICS. 
of vision. This is the angle (pl. 21, fig. 85) formed with each other by 
the two lines, A’a, B’b, drawn between the corresponding extremities of the 
object and its image on the retina. Two objects of different magnitude, as 
AB and A’B’, may appear of the same size when their actual size is 
proportional to their distance from the eye. When the angle is less than a 
certain limit, the object becomes invisible. | 
An image of an object is formed in both eyes; we see but one, however, 
as soon as the eye has been adjusted properly to the distance of the object. 
When the eye is arranged for a distance greater or less than the true one, 
the object will be seen double. In fig. 51, let L and R be the two eyes, A 
and B two objects at different distances from them. If the eyes be fixed 
upon the nearer object, A, the optical axis will be directed towards A, so 
that its image falls in the middle of the retina, at a and a’. The object, A, 
is seen single; B, however, appears double, its image falling out of the 
centre of the retina at b and b’.. The case is reversed when the eyes are 
directed to B. 
Several objects may be seen single by both eyes when their images fall 
on corresponding parts of their retinas. In fig. 52, let L and R again 
represent the two eyes, A, B, and C, three objects before them. All three 
will be seen single, and at the same instant, as their images follow each 
other in the same order in both eyes. 
By irradiation is meant the fact, that a bright object on a dark ground 
appears to us magnified, while a dark object on a bright ground seems to 
be reduced in size. The apparatus represented in figs. 53 and 54 is 
intended to illustrate this phenomenon. fig. 53 represents a piece of 
pasteboard, whose upper half is covered with a piece of white paper, and 
the lower with black. The former is bisected by a narrow strip of black, 
about two lines in breadth, the latter by a strip of white of the same 
breadth, and in the same line with the black strip. On placing the 
pasteboard near a window, the white strip will, at a certain distance, appear 
decidedly broader than the black. 
The following experiment shows that irradiation is not equally strong for 
all persons. Paint upon a piece of white pasteboard two equal, rectangular, 
black spaces, so that the border, al ( fig. 54), shall be about half a line to 
the right, and the border, gh, about the same distance to the left of the 
vertical central line of the pasteboard. If this be observed at a certain 
distance, the edges, a/ and gh, will appear to lie in the same straight line ; 
the precise distance necessary for this result will, however, vary considerably 
for different persons. 
Very small objects on a white ground, vanish entirely when looked at 
under certain conditions, the principal of which is the falling of the image 
on the so-called punctum cecum, that part of the retina at which the optic 
nerve enters. To illustrate this disappearance of an object, lay upon the 
white horizontal surface, nn’( pl. 21, fig. 86), two small dark disks, from 
one to four lines in diameter, and about three inches apart. Bring, now, 
the right eye vertically over the left disk (or the left eye over the righi 
disk), and at a height about five times as great as the distance between the 
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