474 THE SENSOKIAL FUNCTIONS. 



results of the simple combinations of the laws of 

 optics. 



The second class comprehends those in which 

 the cause of deception is more internal, and con- 

 sists in the peculiar condition of the nervous surface 

 receiving the impressions. Ocular spectra of various 

 kinds, impressions on the tongue and the eye from 

 galvanism, and those which occasion singing in the 

 ears, arising generally from an excited circulation, 

 are among the many perceptions which rank under 

 this head. 



The third class of fallacies comprehends those 

 which are essentially mental in their origin, and 

 are the consequences of errors in our reasoning 

 powers. Some of these have already been pointed 

 out with regard to the perceptions of vision and of 

 hearing, the formation of which is regulated by the 

 laws of the association of ideas. But even the 

 sense of touch, which has been generally regarded 

 as the least liable to fallacy, is not exempt from 

 this source of error, as is proved by the well known 

 experiment of feeling a single ball, of about the 

 size of a pea, between two fingers which are 

 crossed ; for there is then a distinct perception of 

 the presence of two balls instead of one. 



But limited as our senses are in their range of 

 perception, and liable to occasional error, we can- 

 not but perceive, that, both in ourselves, and also 

 in every class of animals, they have been studiously 

 adjusted, not only to the properties and the con- 

 stitution of the material world, but also to the 

 respective wants and necessities of each species, in 

 the situations and circumstances where it has been 

 placed by the gracious and beneficent Author of 

 its being.- 



