CONCLUSION. 287 



certain colour, and so on. By tasting 1 we may gain 

 further conceptions of this body. If it is out of reach 

 of our hands, by approaching it we may observe how 

 the apparent size of the body, as the eye shows it 

 to us, increases as we approach. These and many 

 thousand other experiences which we have gained 

 since our earliest youth have gradually put us in a 

 position to form conceptions as to the nature of a body 

 merely from a few sensations. In this act many com- 

 plete inferences are unconsciously involved, so that 

 that which we believe to have been directly perceived 

 is really known by inference from many sensations 

 and from a combination of former experiences. For 

 instance, we think that we see a man at a certain dis- 

 tance ; really, however, we only feel a picture of a 

 certain size of the man on our retina. We know the 

 average size of a man, and we know that the apparent 

 size decreases with the distance ; moreover, we feel 

 the degree of contraction of the muscles of our eye 

 which is necessary to direct the axis of our eye to the 

 object and for the adjustment of our eye to the neces- 

 sary distance. From all these circumstances, the 

 opinion, which we erroneously regard as a direct sensa- 

 tion, is formed. 



10. We have already (chap. iv. 2 ; chap. vii. 

 3) made acquaintance with the methods by which 

 Helmholtz measured the details of the time occupied 

 by the contraction of the muscle and the propagation 

 of the excitement in the motor nerves. By the same, 

 or very similar methods, Helmholtz, and others after 

 him, determined the propagation of the excitement in 

 sensory nerves, and found that it was about 30 m. per 

 second, and therefore, at nearly the same rate as in the 



