But side by side with such advances, springing from analogy and adaptation, there arose in the nineteenth century another and more direct influence of natural science, as previously mentioned. In its natural progress scientific research came to touch upon psychological problems at several points, and since it laid hold of them and followed them out for its own ends, it immediately became a pioneer for psychology. The first and at the same time the strongest of these impulses came from the advance of the physiology of the senses. In the fourth decade of the nineteenth century remarkably active and fruitful work in this field began. Physiologists and physicists vied with each other in accurate study of the structure and functions of sense organs. Naturally they were not able to stop at the material functions in which they were most directly interested. They could not forbear to draw into the circle of their investigations those mental functions mediated by the physiological functions and explainable on a physiological basis. The eye, especially, attracted scores of investigators, both because it is very richly endowed with dioptric and mechanical auxiliary apparatus and because it is particularly important on account of the delicacy and diversity of its functions. Yet cutaneous sensations and hearing were not neglected. Johannes Müller, E. H. Weber, Brewster, and above all--especially versatile, far-seeing, and inventive--the somewhat younger Helmholtz, are only a few of the most noteworthy representatives of this class of research. They brought to psychology results such as it had never known before--results resting on well-conceived and original questions as to the nature of things, and on skillful attempts at arranging the circumstances for an answer, that is, on _experiment_ and when possible on exact _measurement_ of the effects and their causes. When Weber in 1828 had the seemingly petty curiosity to want to know at what distances apart two touches on the skin could be just perceived as two, and later, with what accuracy he could distinguish between two weights laid on the hand, or how he could distinguish between the perception received through the muscles in lifting the weights and the perception received through the skin, his curiosity resulted in more real progress in psychology than all the combined distinctions, definitions, and classifications of the time from Aristotle to Hobbes. The surprising discovery of hitherto unknown sense organs, the muscles and the semicircular canals, was made at that time, although not thoroughly verified until later. That discovery meant not only an increase of knowledge, but also a widening of the horizon, since the most conspicuous peculiarity of these organs is that they do not, like the others, bring to our consciousness external stimuli in the ordinary sense, but processes on the inside of the body.