628 THE SENSES. 



position of the two eyes. The ostrich, with the head directed forward, 

 can easily see objects placed a few yards behind its back ; and in many 

 fish, when examined from different points in an aquarium, it is impossi- 

 ble for the observer to place himself in any position, above, behind, or 

 on either side, where he cannot see one or both of the pupils of the 

 animal. The field of vision consequently, for the animal, is a complete 

 sphere ; the light being perceptible from every point of the surrounding 

 space. In man, the external borders of the field of vision are very ill 

 defined ; and objects placed at a lateral distance of 90 degrees must be 

 very brilliant to attract attention. For practical purposes, the space 

 within which objects are perceptible is one of not more than 75 degrees 

 on each side, or 150 degrees for the entire field of vision. 



Line of Direct Vision. Within the field of vision, however, there is 

 onlyjone point, at its centre, where the form of objects can be perceived 

 with distinctness ; and the prolongation of this point, in the visual axis 

 of the eye, from the pupil forward, is called the "line of direct vision." 

 Objects met with upon this line can be distinctly seen ; all others, situ- 

 ated upon either side, above or below it, are perceived only in an imper- 

 fect manner. If the observer place himself in front of a row of vertical 

 stakes or palisades, he can see those placed directly in front of the e3'e 

 with perfect distinctness ; but those on each side appear as uncertain 

 and confused images. On looking at the middle of a printed page, in 

 the line of direct vision, we see the distinct outlines of the letters; 

 while at successive distances from this point, the eye remaining fixed, 

 we distinguish first only the separate letters with confused outlines, 

 then only the words, and lastly only the lines and spaces. 



This limitation of serviceable sight to the line of direct vision is 

 practically compensated by the great mobility of the eyeball, which 

 turns successively in different directions ; thus shifting the field of vision 

 and examining in turn every part of the space attainable by the eye. 

 In reading a printed page, the eye follows the lines from left to right, 

 seeing each letter and word distinctly in succession. At the end of 

 each line, it returns suddenly to the commencement of the next, repeat- 

 ing the same movement from the top to the bottom of the page. 



The deficiency of distinctness outside the line of direct vision depends 

 upon two causes, which are both present, although either separately 

 would tend to produce a similar result ; namely, 1st, inaccurate focus- 

 ing of the luminous rays ; and 2d, diminished acuteness of the retinal 

 sensibility. 



Rays of light entering the eye from the front, in the line of direct 

 vision, may be brought to an accurate focus at the situation of the 

 retina. But those which enter at a certain degree of obliquity, whether 

 from above, from below, or from one side, suffer a more rapid conver- 

 gence and are accordingly brought to a focus and again dispersed, before 

 reaching the retina. Thus rays diverging from the point a (Fig. 198), 

 in the line of direct vision, are again concentrated at x, and form a dis- 

 tinct image upon the retina at that point. But those coming from 6, 



