OF THE VOICE AND SPEECH. 



553 



ness of the circle of light thus formed. By experiments of this kind, 

 the time has been found to vary, in different individuals, or in diffe- 

 rent states of the same individual, from about 1-4 th to 1-1 Oth of a 

 second : that is, the impression must be repeated from four to ten times 

 in each second, to insure the continuousness of the image. 



971. The impressions of variety of colour, are produced by the diffe- 

 rently-coloured rays, which objects reflect or transmit to the eye. It 

 is curious that some persons, whose sight is perfectly good for forms, 

 distances, &c., are unable to discriminate colours. This curious affec- 

 tion has received the name of Daltonism ; from the circumstance that the 

 celebrated Dalton was an example of it. There are numerous modifi- 

 cations of it ; the want of power to discriminate colour being total in 

 some ; whilst in others it extends only to certain shades of colour, or to 

 the complementary colours. 



972. When the retina has been exposed for some time to a strong 

 impression of some particular kind, it seems less susceptible of feebler 

 impressions of the same kind ; thus if we look at any brightly -luminous 

 object, and then turn our eyes upon a sheet of paper, we shall perceive 

 a dark spot upon it ; the portion of the retina, which had received the 

 brighter image, not being affected by the fainter one. Again, when 

 the eyes have received a strong impression from a coloured object, the 

 spot which is seen when the eyes are directed upon a white surface ex- 

 hibits the complementary colour ; for the retina has been so strongly 

 affected, in the part that originally received the image, by its vivid hue, 

 that it does not perceive the fainter hue of the same kind in the object 

 to which it is then turned, and it is impressed only by the remaining 

 rays forming the complementary colours. This explanation applies to 

 the phenomena of the coloured shadows which are often seen at sunset, 

 and of those which may be seen in a room whose light enters through 

 coloured glass or drapery. For if the prevailing light be of one colour, 

 orange or red for instance, the eye will not take cognizance of that 

 colour in the faint light of the shadows ; and will see only its comple- 

 ment, blue or green. If the shadow be viewed through a tube, in such 

 a manner that the general coloured ground is excluded, it presents the 

 ordinary tint. 



CHAPTER XIV. 



OF THE VOICE AND SPEECH. 



973. THEEE is one particular application of Muscular power in Man, 

 which deserves special consideration, as being that by which he effects 

 his most complete and intimate communication with his fellows ; that, 

 namely, by which his organ of Voice is put into action. In all air- 

 breathing Vertebrata, the production of sound depends upon the pas- 

 sage of air through a certain portion of the respiratory tubes, which is 



