Another point to be very careful about is the matter of uninterrupted straight lines. For instance, the small diagrams on pages [22] and [23] show two pairs of perfectly straight up-and-down parallel lines. This is probably hard for you to believe, since the lines in the right-hand figure seem to definitely bulge outwards. However, careful measurement with your ruler or a pencil will prove to you that the lines actually are as straight as those in the figure on the left. These latter, however, appear straight because they are uninterrupted and unbroken. Those at the right appear to bulge outwards merely because there are so many radiating lines running through them.
Applying this principle to clothes, you can easily see that the tall, slender effect you hoped to gain by the straight up-and-down lines of your costume may be entirely ruined if you apply trimmings of any kind which radiate outwards toward these lines. The dresses shown on pages [22] and [23] will prove this to you. The woman at the left with her uninterrupted, harmonious, gracefully flowing up-and-down lines looks taller, slenderer, more dignified and in every way more pleasing than the woman at the right, the radiating lines of whose gown make her figure seem to bulge outwards in a most discouraging manner.
Another striking example of optical illusion showing one reason why some look stouter than they really are is shown in the illustrations on pages [26] and [27]. As in the previous examples, the two figures (diamond shape figures in this case) are, by actual measurement, exactly the same size. The horizontal line across the one at the left, however, makes it appear much wider than the one at the right with the vertical line through the center.
Now study the clothes of the two women which illustrate these illusions. Both women are holding their arms so as to give their figures a sort of diamond shape. The one at the left, however, by her broad, drooping hat, her large, bulky fur stole, the large-figured material of her tunic, and especially by the horizontal, or nearly horizontal lines of her neck, her girdle, and the band of fur on her skirt, gives herself the appearance of conspicuous stoutness.
The middle lines in the two small diagrams are the same length. But on the left, shorter accompanying lines seem to shorten the one between. On the right longer accompanying lines seem to lengthen the one between.
Now see how the woman at the left has unknowingly emphasized her stoutness while the one at the right has properly gained a slender effect by using trimming in accordance with the principles of these optical illusions.
On the other hand, the woman at the right has designed her costume entirely on the principles of vertical lines. The tall hat with its appropriate trimming, the long, simple lines of her collar, her neck-piece, the row of tiny buttons down the front of her dress, and indeed the lines of the dress itself all conspire to give her the appearance of height, smartness, and slenderness.