A Comparison of Sight and Touch

OUR knowledge of the position of objects in the external world is derived almost wholly through the senses of sight and touch, each sense aiding the other, aind both receiving assistance from the muscular sense. Without attempting a discussion of the question whether touch or sight furnishes the more primitive and fundamental data to consciousness, it is interesting to inquire which sense supplies us with the more accurate information as to the position of objects around us. The comparison may be made in several ways. In the first place, we may compare the smallest distances within which two impressions made upon sensitive surfaces can be recognized as separate and distinct. It has been found, for example, that the distance between two luminous points, as two fixed stars, must subtend a visual angle of at least one minute, in order that the sources of light may be recognized as distinct from one another'. A visual angle of one minute corresponds to a distance of O000438 mm. on the retina, which is about the diameter of the cones in the macula lutea according to K o 11 i k e r 's measurements. Now, since the smallest distance upon the surface of the body at which two tactile impressions are recognized as distinct is, according to Weber's researches, 11 mm.2 (upon the tip of the tongue), it follows that the retina is 251 times more accurate than the most sensitive part of the surface of the body in localizing impressions made upon it. It may be objected, however, that a comparison of this sort has little value, inasmuch as it is the optical image of the object which is applied to the retina, while the object itself is brought in contact with the surface of the body. It is interesting, therefore, to compare the absolute size of the smallest intervals by which two external objects must be separated in order that they may make distinct impressions upon the two senses. In