the eyes, and it will be perceived that the space be- tween the two thumbs appears longer than the black line, and before the rule reaches the eyes, the differ- ence will be nearly as three to one.
This will lead the pupil to conclude, that objects of the same size appear the smaller the farther they are from the eye, and the larger the nearer they are to it, though the real size of the object is unchanged.
fig. 1. plate ii.
The apparent size of an object depends upon the an- gle formed by lines drawn from the extremities of an object to the eye, the eye forming the apex of the an- gle. Thus in fig. 1, let the circle represent the eye. Three arrows of equal length are placed at different distances from it. Lines are drawn from both extremi- ties of each arrow, so as to cross each other at the same point in the eyes, and continue on till they touch the back of the eye. The pupil has been informed (p. 4.) that it is not the length of the sides, but the opening of the sides which determines the size of the angle. If the pupil will measure the angle, which the lines from the extremities of the nearest arrow to the eye form, (see page 28,) he will find it to consist of about 92 de- grees, which are more than quarter of a circle. If he will then measure the angle formed by the lines from the second arrow, he will find it to consist of about 27 degrees. Finally, if he measure the angle of the third arrow, the farthest from the eye, he will find it to consist of only 14 degrees.
The lines of any angle if continued beyond the apex, as in this case, will form a second angle of the same size as the first. Of course the linds of the first angle, being extended beyond the apex to the back of the eye, form there a corresponding angle of 92 degrees, and
