If two angles of equal size are bounded by small and large angles respectively, the apex in each case being common to the inner and two bounding angles, the effect of contrast is very apparent, as seen in Fig. 20. In Fig. 57 are found examples of effects of lines contrasted as to length. [Illustration: Fig. 21.--Owing to perspective the right angles appear oblique and vice versa.] The reader may readily construct an extensive variety of illusions of contrast; in fact, contrast plays a part in most geometrical-optical illusions. The contrasts may be between existing lines, areas, etc., or the imagination may supply some of them. [Illustration: Fig. 22.--Two equal diagonals which appear unequal.] _Illusions of Perspective._--As the complexity of figures is increased the number of possible illusions is multiplied. In perspective we have the influences of various factors such as lines, angles, and sometimes contour and contrast. In Fig. 21 the suggestion due to the perspective of the cube causes right angles to appear oblique and oblique angles to appear to be right angles. This figure is particularly illusive. It is interesting to note that even an after-image of a right-angle cross when projected upon a wall drawn in perspective in a painting will appear oblique. [Illustration: Fig. 23.--Apparent variations in the distance between two parallel lines.] A striking illusion involving perspective, or at least the influence of angles, is shown in Fig. 22. Here the diagonals of the two parallelograms are of equal length but the one on the right appears much smaller. That _AX_ is equal in length to _AY_ is readily demonstrated by describing a circle from the center _A_ and with a radius equal to _AX_. It will be found to pass through the point _Y_. Obviously, geometry abounds in geometrical-optical illusions. [Illustration: Fig. 24.--A striking illusion of perspective.] The effect of contrast is seen in _a_ in Fig. 23; that is, the short parallel lines appear further apart than the pair of long ones. By adding the oblique lines at the ends of the lower pair in _b_, these parallel lines now appear further apart than the horizontal parallel lines of the small rectangle. The influence of perspective is particularly apparent in Fig. 24, where natural perspective lines are drawn to suggest a scene. The square columns are of the same size but the further one, for example, being apparently the most distant and of the same physical dimensions, actually appears much larger. Here is a case where experience, allowing for a diminution of size with increasing distance, actually causes the column on the right to appear larger than it really is. The artist will find this illusion even more striking if he draws three human figures of the same size but similarly disposed in respect to perspective lines. Apparently converging lines influence these equal figures in proportion as they suggest perspective.