ox THE JIECIIANISM OF THE EYfi. 



.575 



sively affected by sounds in different direc- 

 tions. Supposing the rye capable of con- 

 veying a distinct idea of t\Vo points subtend- 

 ing an angle of e^ minute^ which is, perhaps, 

 nearly the smallest interval at which two ob- 

 jects can be distinguished, although a line, 

 subtending onl*one tenth of a minute in 

 bre&dth, may sometimes be perceived as a 

 single object ; there must, on this supposi- 

 tion, be about 36O thousand sentient points, 

 for a field of view of 10 degrees in diameter, 

 and above 60 millions for a field of 140 de- 

 grees. But, on account of the various sen- 

 sibility of the retina, to be explained here- 

 after, it is not necessary to suppose, that 

 there are more than 10 million sentient 

 points, nor can there easily be less than one 

 million : the optic nerve may, therefore, be 

 judged to consist of several millions of dis- 

 tinct fibres. By a rough experiment, I find, 

 that I can distinguish two similar sounds 

 proceeding from points which subtend an 

 angle of about five degrees. But the eye can 

 discriminate, in a space subtending every way 

 five degrees, about 90' thousand different 

 points. Of such spaces, there are more than a 

 thousand in ahemisphere : so that the ear can 

 convey an impression of about a thousand 

 different direetions. The ear has not, how- 

 ever, in all cases, quite so nice a discrimi- 

 nation of the directions of sounds : the rea- 

 son of this difference between the eye and 

 ear is obvious ; each point of the retina has 

 only three principal colours to perceive, since 

 the rest are probably composed of various 

 proportions of these ; but there being many 

 thousands or millions of varieties of sound 

 audible in each direction, it was impossible 

 that the number of distinguishable directions 

 should be very large. It is not absolutely cer- 

 tain, that every part of the auditory nerve is 



capable of receiving the impression of each 

 of the very great diversity of tones that we 

 can distinguish, in the same manner as each 

 sensitive point of the retina receives a dis- 

 tinct impression of the colour, as well' as of 

 the strength, of the light which falls on 

 it ; although it is extremely probable, that 

 all the different parts of the surface, ex- 

 posed to the fluid of the vestibule, are 

 more or less affected by every sound, but 

 in different degrees and succession, accord- 

 ing to the direction and quality of the vibra- 

 tion. Wl^ether or no, strictly speaking, we 

 can hear two sounds, or see two objects, in 

 the same instant, cannot easily be determined; 

 but it is sufficient, that we can do both, with- 

 out the intervention of any interval of time 

 perceptible to the mind ; and indeed we could 

 form no idea of magnitude, without a com- 

 parative, and therefore nearly cotemporary, 

 perception of two or more parts of the same 

 object. The extent of the field of perfect vi- 

 sibn, for each position of the eye, is certainly 

 not very great ; although it will appear here- 

 after, that Its refractive powers are calcu- 

 lated to take in a moderately distinct view of 

 a whole hemisphere : the sense of heaiing is 

 equally perfect in almost every direction. 



IV. Dr. Porterfield has applied an ex- 

 periment, first made by Scheiner*, to the de- 

 termination of the focal distance of the eye ; 

 and has described, under the name of an op- 

 tometer, a very excellent instrument, founded 

 on the principle of the phenomenon f. But 

 the apparatus is capable of considerable im- 

 provement ; and 1 shall beg leave to de- 

 scribe an optometer, simple in its construc- 

 tion, and equally convenient and accurate in 

 its application. 



• Priestley's opt. 113. 



t Edinb. Med. Ess. IV. I8i. 



