Yet a psychological laboratory looks much like any other modern laboratory. Around the rooms run glass-cases filled with fine instruments. Shelves line up, row after row, of specimen-jars and bottles. Charts cover the remainder of the walls. The tables and floors are crowded with working apparatus. Two large rooms and one small one are now occupied at Harvard. Four more rooms will be added to these this summer. Also, the spirit that reigns in these rooms is the same that is found in other laboratories of exact science. This is the important thing. The minds of these workers are not wandering in dialectics and vagrant hypotheses. Reverence has opened her eyes. Hypotheses they have, and must have. Often they hold conflicting opinions. But the referee is always present--Nature herself. To experiment, to show the fact, is always the method of debate. This is the great advantage of the modern way of studying psychology over the old. The American public is so practical that I feel I can alone satisfy its "whats and wherefores" by explicitly describing some of the investigations being carried on here. EFFECT OF ELEMENTARY SENSATIONS ON ONE ANOTHER. Here is a lantern throwing a steady light through a large tube. (See illustration below, the right hand group.) By transparent slides of colored glass or gelatine, the light may be made of any color. At the end of the tube is a box, like a camera. The operator covers his head with a cloth, and observes the color of the light as it shines from the tube through, or on, a tiny hole in the dark box. The size of the hole can be varied by moving slides, worked by micrometer screws so fine that they measure the dimensions of the hole to the four-hundredth of an inch. [Illustration: STUDYING THE EFFECTS OF SOUND AND OF ATTENTION ON COLORS.] The first step is to discover the "threshold" of each separate color. That means the smallest-sized hole through which each color can be distinguished. This varies for different colors. But now comes the interesting point. The size of the hole, for any given _color seen_, varies according to the nature of any _sound heard_ at the same time. For instance, in order to distinguish a given red, the hole must be larger or smaller, in proportion as the pitch of a musical tone is lower or higher, fainter or stronger. [Illustration: STUDYING THE EFFECTS OF COLORS ON JUDGMENTS OF TIME.] The above experiment is one in a system of investigations, intended to discover the laws by which the simplest sensations modify each other under the simplest conditions. These are laws as fixed as the laws of gravity, and, once determined, we may move on to study the combination of these elements into the higher thought processes. EFFECTS OF ATTENTION.