H46 



VISION. 



mosphere and a sufficient amount of light are 

 of importance, for every one must be aware 

 how deceptive is the estimate of the size of an 

 object when seen through a fog, or looming 

 large in the gloom of the evening. 



Some persons are gifted, as it were, intui- 

 tively with the power of judging correctly of 

 the true dimensions of objects. Napoleon 

 possessed this in an eminent degree. He 

 could tell at a glance the number of men com- 

 posing a distant mass of troops, and the space 

 of ground they would cover when deployed. 

 In doing this a mental computation must have 

 taken place unconsciously, rapid as thought, 

 and based on a combination of great powers of 

 calculation, long experience, and profound 

 knowledge of the subject. 



For the production of a distinct image on 

 the retina it is necessary that it be of a certain 

 magnitude, which will depend on the suscep- 

 tibility of the eye. We may here observe that 

 the apparent magnitude of an object must not 

 be confounded with its apparent superficial 

 magnitude, the term being applied to its linear 

 magnitude. The apparent superficial magni- 

 tude varies in proportion to the squares of the 

 apparent magnitude. The image of an object 

 moderately illuminated must be O'OOl of an 

 inch long, or the extreme rays of light must 

 form an angle of half a minute in the eye at a 

 minimum ; whence it follows that an object 

 of mean illuminating power will be visible if 

 its distance is not greater than 68,000 or 69,000 

 times its greatest length. Strongly luminous 

 bodies, as the fixed stars, are visible at infi- 

 nitely small visual angles. They excite in the 

 eye merely a sensation of light, without creat- 

 ing any impression as to their apparent mag- 

 nitude. The disk of the moon subtends a 

 visual angle of half a degree, the diameter 

 of its picture on the retina will be ^^ of 

 an inch, and the entire superficial magnitude 

 of the image the -52oo tn of a square inch ; 

 yet forms of light and shade are perceptible 

 whose linear dimensions occupy upon the re- 

 tina a space whose diameter does not exceed the 

 Tooocyocy of a square inch. The eye in a healthy 

 condition is capable of reading print in the 

 light of the full moon, and that of the noon- 

 day sun, their intensities being to each other as 

 1 to 300,000. Plateau asserts that white may 

 be distinctly seen in the light of the sun at ail 

 angle of 12", yellow at an angle of 13", red at 

 23", and blue at 26", but that in ordinary day- 

 light these angles must be half as large again. 



In estimating the motions of objects, we are 

 guided by the movement of their images on 

 the retina; and unless a body moves in such 

 a manner that the line of vision shall describe 

 at least one degree in each minute of time, its 

 motion will not be perceptible ; for which 

 reason we are not conscious of the move- 

 ments of the heavenly bodies. The more 

 nearly at right angles to the line of vision the 

 direction of the motion is, the greater will be 

 the apparent motion produced by any real 

 movement of an object. 



Erect Vision. A variety of explanations 

 have been offered to account for objects 



not being seen reversed, according to the 

 position in which they are depicted on the 

 retina ; but it would appear that by many 

 a sufficient distinction has not been drawn 

 between seeing the image and seeing by 

 means of it. A little reflection shows that 

 the actual perception of the object takes 

 place in the sensorium, and that the image on 

 the retina is only a necessary step in the pro- 

 cess. In truth, we have no notion of upright 

 or inverted, except that which is founded on 

 experience. A man is upright whose head is 

 upwards, and his feet downwards. Whatever 

 be our standard of up or down, the sensible 

 representation of up will always be an image 

 moving on the retina towards the lower side, 

 and the sensible representation of down will 

 be a motion towards the upper side. The 

 head of the man's image is towards the image 

 of the sky ; its feet are towards the image of 

 the ground ; and consequently it cannot ap- 

 pear otherwise than upright. So, as all objects 

 are inverted on our retinae, they do not change 

 their relation one with another, and our only 

 knowledge of position is from relative rela- 

 tion, therefore we may truly say that we do 

 not see the image on the retina, but by means 

 of it. For every image on the retina, we sub- 

 stitute an object, and seek in a definite direc- 

 tion for the object corresponding to a definite 

 image on the retina. In this we are assisted 

 by other perceptions of sense, there existing 

 the greatest harmony between such percep- 

 tions in respect to locality. 



Dr. Alison, in an able paper*, advances the 

 opinion, that the harmony between the inti- 

 mations acquired by sight and by touch, as to 

 the relative position of objects or their parts, 

 notwithstanding that the impressions made 

 by them on the external organs of sight and 

 of touch are arranged inversely in regard to 

 one another, arises from the course of the 

 optic nerves and tractus optici, whereby im- 

 pressions on the upper part of the retina are 

 in fact impressions of the lower part of the op- 

 tic lobes that is to say, of the sensorium 

 and impressions on the outer part of the retina 

 are, in like manner, on the inner part of the 

 sensorium. This theory was first suggested by 

 Mr. Dick, a veterinary surgeon ; but though 

 ingenious, can hardly be considered satisfac- 

 tory, as it implies the necessity for con- 

 ditions which cannot always be fulfilled : and 

 truly the question is of a nature not to be 

 decided merely by anatomical inquiry. 



Kepler's explanation of objects appearing 

 erect to us is, that the mind, perceiving the 

 impulse of a ray on the lower part of the re- 

 tina, conceives this ray to be directed from a 

 higher part of the object, and vice versa. Por- 

 terfield argues that the mind never sees any 

 picture painted on the retina, and consequently 

 never judges of the object from what it ob- 

 serves in the picture ; and that in seeing any 



* On Single and Correct Vision, by means of 

 Double and Inverted Images on the Retinae, Trans- 

 actions of the Royal Society of Edinburgh, vol. xiii. 

 p. 472. 



