Figure 1 shows two lines of equal length, yet the vertical line will to most persons seem longer than the horizontal one.

In Figure 2 the lines A and B are of the same length, yet the lower seems much longer.

Those things look smallest over which the eye moves with least resistance.

In Figure 3, the distance from A to B looks longer than the distance from B to C because of the time we involuntarily take to notice each dot, yet the distances are equal.

For the same reason, the hatchet line (A–B) appears longer than the unbroken line (C–D) in Figure 4, and the lines E and F appear longer than the space (G) between them, although all are of equal length.

Filled spaces look larger than empty ones because the eye unconsciously stops to look over the different parts of the filled area, and we base our estimate upon the extent of the eye movements necessary to take in the whole field. Thus the filled square in Figure 5 looks larger than the empty one, though they are of equal size.

White objects appear much larger than black ones. A white square looks larger than a black one. It is said that cattle buyers who are sometimes compelled to guess at the weight of animals have learned to discount their estimate on white animals and increase it on black ones to make allowances for the optical illusion.

The dressmaker and tailor are careful not to array stout persons in checks and plaids, but try to convey an impression of sylph-like slenderness through the use of vertical lines. On the other hand, you have doubtless noticed in recent years the checkerboard and plaid-covered boxes used by certain manufacturers of food products and others to make their packages look larger than they really are.

The advertiser who understands sensory illusions gives an impression of bigness to the picture of an article by the artful use of lines and contrasting figures. If his advertisement shows a picture of a building to which he wishes to give the impression of bigness, he adds contrasting figures such as those of tiny men and women so that the unknown may be measured by the known. If he shows a picture of a cigar, he places the cigar vertically, because he knows that it will look longer that way than if placed horizontally.

A subtle method of conveying an idea of bigness is by placing numbers on odd-shaped cards or blocks, or on any blank white space. The object or space containing the figures always appears larger than the corresponding space without the figures.

This fact has been made the basis of a psychological experiment to determine the extent to which a subject's judgment is influenced by suggestion. To perform this experiment cut bits of pasteboard into pairs of squares, circles, stars and octagons and write numbers of two figures each, say 25, 50, 34, 87, etc., upon the different pieces. Tell the subject to be tested to pick out the forms that are largest. The susceptible person who is not trained to discriminate closely will pick out of each pair the card that has the largest number upon it.

This test can be made one of a series used in examining applicants for commercial positions. It can also be used to discover the weakness of certain employees, such as buyers, secretaries and others who are entrusted with secrets and commissions requiring discretion, and who must be proof against the deceptions practiced by salesmen, promoters and others with seductive propositions.

This examination can be carried still further to test the subject's credulity or power of discrimination. What is known as the "force card" test was originally devised by a magician, but has been adopted in experimental psychology. Take a pack of cards and shuffle them loosely in the two hands, making some one card, say the ace of spades, especially prominent. The subject is told to "take a card." The suggestive influence of the proffered card will cause nine persons out of ten to pick out that particular card.

Turning from illusions of suggestion, shape and size, another field of peculiar sensory illusions is found in color aberration. Some colors look closer than others. For instance, paint an object red and it seems nearer than it would if painted green.

Aside from the obvious uses to which these sense-illusions can be put, they form the basis for a number of psychological experiments to test the abilities of persons in many ways. Here is a test which deals with the range of attention. If you desire to discover the capacity of any person to pay attention to unfamiliar questions or subjects which might at some future time have great importance, try this test. Have a piece of pasteboard cut into squares, circles, triangles, halfmoons, stars and other forms. Then write upon each piece some such word as hat, coat, ball or bat. The objects are then placed under a cloth cover and the subject to be examined is told to concentrate his attention on the shapes alone, paying no attention to the words. The cloth is lifted for five seconds and then replaced. The subject is then told to draw with a pencil the different shapes and such words as he may chance to remember. The experiment should then be repeated, with the injunction to pay no attention to the shapes but to remember as many words as possible, and write them down on such forms as he may happen to recall.

Of course, the real object is to determine whether the subject will see more than he is told, or whether he is a mere automaton. The result will tell whether his attention is of the narrow or broad type. If it be narrow, he will see only the forms in the first case and no words, and in the second case he will remember the words but be unable to recall the shape of the pieces of cardboard.

His breadth of attention will be shown by the number of correct forms and words combined which he is able to remember in both cases. In other words, this will measure his ability to pay attention to more than one thing at a time.

Other things being equal, the narrow type of attention belongs to a man fitted for work as a bookkeeper or mechanic, while the broad type of attention fits one for work as a foreman or superintendent or, lacking executive ability, for work requiring the supervision of mechanical operations widely separated in space.

The ordinary man sees but one thing at a time, while the exceptional man sees many things at every glance and is prepared to remember and act upon them in emergency.

Having determined a person's scope of attention, you may want to test his accuracy in details as compared with other men. To conduct such an experiment dictate a statement which will form one typewritten letterhead sheet. This statement should comprise facts and figures about your business of which the subjects to be tested are supposed to have accurate knowledge. After this original page is written, have your typist write out another set of sheets in which there are a large number of errors both in spelling and figures. Then have each of the persons to be examined go through one of these sheets and cross out all the wrong letters or figures. Time this operation. The man who does it in the quickest time and overlooks the fewest errors, naturally ranks highest in speed and accuracy of work.

Look into your own business and you will undoubtedly find some department, whether it be store decoration, office furnishing, window dressing, advertising, landscape work or architecture, in which a systematic application of a knowledge of sensory illusions will produce good results.