ERECT VISION. 453 



media of the eye, suffer a slight change in their course, pass outwards 

 to the object, and correspond with the direction in which the central 

 ray of each luminous pencil (Fig. 83) reaches the eye, as it proceeds 

 from the object to the retina. In applying this theory to the expla- 

 nation of the problem of erect vision from an inverted image, it is sup- 

 posed that every point of the object (of the arrow, for example) being 

 seen along these lines of visual direction, appears to the mind in its 

 true position in space, and that hence the entire object is perceived 

 erect. According to Mu'ller, however, images of objects formed on 

 the retina, may really be perceived by the mind in their inverted posi- 

 tion, but as all objects, including the body and limbs themselves, are 

 thus seen inverted, their relation to each other remains unaltered, and 

 we should be ignorant of this inversion, were we unacquainted with 

 the laws of optics. Some have supposed that the sense of touch cor- 

 rects a primitive error of ocular observation, or of perception, in the 

 infant ; but to this it may be replied, that the one sense is always in 

 harmony with the other, the image of the hand being inverted, and 

 thrown upon the same part of the retina, as the image of the object to 

 which it is directed. Moreover, it is obvious that the general sense 

 of locality or position of the body, gives rise to the notions and terms, 

 upper and lower, above and below, right and left, and so we regard 

 things as erect or inverted, according to their position in relation to 

 that of our own body. In this way also, we know that to look at 

 something above us, we must turn our head or eyes upwards, and such 

 a movement, not the position of the retinal image, which is unknown 

 to us, determines our notion or judgment of the position in space, in 

 which the object lies as regards our body. 



The area of outward visual projection, is named the field of vision ; 

 its horizontal and vertical measurements have been differently esti- 

 mated at from 116 to 120 and from 130 to 180 of a circle respect- 

 ively. Its greater horizontal diameter is owing to the two eyes being 

 concerned in its production. The horizontal diameter is not constant, 

 but diminishes in convergence of the two eyes. Lines, drawn from 

 the upper and lower, and from the lateral boundaries of this area, to 

 the optic centre of the eye, form angles at that point ; when prolonged 

 backwards to the retina, they also form similar angles. These are 

 the optic or visual angles of the field of vision. 



The only part of the field in which vision is perfectly distinct, is a 

 small surface, the centre of which corresponds with the hinder end of 

 a line drawn from the yellow spot to the centre of the cornea, i. ?., 

 along the visual, or optic axis of the eye. The area of the retinal sur- 

 face which is best fitted for distinct vision, is about J or J of a line in 

 diameter ; this corresponds with the breadth of the yellow spot. 

 Around this, is a small circular portion of the retina, known as the 

 circle of indirect vision, and beyond that area, vision becomes less dis- 

 tinct, in proportion to the distance of the retinal image from it. The 

 circle of indirect vision is said to be increased during near vision. 

 We can see distinctly, at one time, about six or eight letters of ordi- 

 nary type ; but the lines immediately above and below, are indistinct. 

 Objects, the images of which are situated at an angle of 50 or 60 



