544 OF SENSATION. 



impulses per second, the pitch of which is about four octaves above the 

 highest F of the piano-forte. On the other hand, no sequence of vibra- 

 tions fewer than 7 or 8 in a second, can produce a continuous tone, 

 because the impression left by each impulse has passed away, before the 

 next succeeds ; and there is consequently nothing more than a succession 

 of distinct beats. The strength or loudness of musical tones depends 

 (other things being equal) on the force and extent of the vibrations, com- 

 municated by the sounding body to the medium which propagates them. 

 This will diminish, however, with distance, which softens loud tones by 

 lowering the intensity of the undulations, as a consequence of their more 

 extensive diffusion. The causes of the difference in the timbre, or 

 quality of musical tones, such, for instance, as those which exist 

 between the tones of a flute, a violin, a trumpet, and a human voice, 

 all sounding a note of the same pitch, are unknown : but they pro- 

 bably depend upon differences of form in the undulations. Our ideas 

 of the direction and distance of sounds, are for the most part formed by 

 habit. Of the former we probably judge in great degree, by the rela- 

 tive intensity of the impressions received by the two ears ; though we 

 may form some notion of it by a single ear, if the idea just stated as 

 to the use of the semicircular canals ( 952), be correct. Of the dis- 

 tance of the sounding body, we judge by the intensity of the sound, 

 comparing it with that which we know the same body to produce when 

 nearer to us. The Ear may be deceived in this respect as well as the 

 eye ; thus the effect of a full band at a distance may be given by the 

 subdued tones of a concealed orchestra close by us ; and the Ventrilo- 

 quist produces his deception, by imitating as closely as possible, not 

 the sounds themselves, but the manner In which they would strike our 

 ears. 



6. Of the Sense of Sight. 



955.. By the faculty of Sight we are made acquainted, in the first 

 place, with the existence of Light ; and by the medium of that agent, 

 we take cognizance of the form, size, colour, position, &c., of bodies 

 that transmit or reflect it. As to the mode in which luminous impres- 

 sions are propagated through space, philosophers are at present unde- 

 termined ; and the question is of no physiological importance, since all 

 are agreed as to the laws which regulate their transmission. These 

 laws, which will be found at large in any Treatise on Natural Philoso- 

 phy,* may be briefly stated as follows. 



I. Light travels in straight lines, so long as the medium through 

 which it passes is of uniform density. 



II. When the rays of light pass from a rarer medium into a denser 

 one, they are refracted towards a- line drawn perpendicularly to the 

 surface they are entering. 



III. When the rays of light pass from a .denser medium into a rarer 

 one, they are refracted from the perpendicular. 



IV. When^rays proceeding from the several points of a luminous 

 object, at a distance, fall upon a double convex lens, they are brought 



* See Dr. Golding Bird's Manual, Chap. XXII. 





