7x8 



TEXT-BOOK OF PHYSIOLOGY 



red and clearly defined, the periphery blue and ill-defined. The reason 

 for this is clear. The eye more readily accommodates itself for the 

 red rays, and hence their focal point is distinct. The blue rays, having a 

 higher degree of refrangibility, come to a focus, cross and diverge, and give 

 rise to diffusion-circles. If a biconcave glass be placed before the cobalt, 

 the blue rays can be focalized on the retina, while the red will fall on the retina 

 without focalization. The image will now be blue and distinct in the center, 

 the periphery red and ill-defined. With the removal of the minus glass the 

 reverse condition again obtains. 



Imperfect Centering. From a purely physical point of view, the eye 

 is not a perfect optic instrument. In addition to the defects noticed in the 

 foregoing paragraphs, there is yet another, viz.: an imperfect centering of 

 the refracting surfaces. In first-class optic instruments the lenses are 

 centered that is, their optic centers are situated on the same axis. In 

 viewing an object through such a system the visual line corresponds with 

 the axis of the lens system. This is not the case with the refracting system 



FIG. 307. DIAGRAM SHOWING THE CORNEAL Axis D D, THE OPTIC Axis O A, THE VISUAL 



AXIS V L, AND THE LlNE OF FIXATION V C', ALSO THE THREE ANGLES, , 0, U. 



of the eye. A line passing through the center of the cornea and the center 

 of the eye, the optic axis (O A in Fig. 307), does not pass exactly through 

 the center of the lens and does not fall into the point for most distinct vision, 

 the fovea. This has lead to the recognition of other lines the relations of 

 which must be kept in mind in all optic discussions, viz. : 



1. The visual axis or visual line (V L), the line connecting the point viewed, 



the nodal point, and the fovea centralis. 



2. The line of fixation or line of regard (V C), the line connecting the point 



viewed with the center of rotation, the latter being situated 6 mm. behind 

 the nodal point of the eye and 9 mm. before the retina. The relation 

 of these lines and certain angles connected with them are shown in Fig. 

 307. The angle included between the line D D (the major axis of the 

 corneal ellipse) and the visual line is the angle alpha, amounting on the 

 average to 5. The angle incuded between the optic axis and the line of 

 fixation or regard is the angle gamma, while the angle between the optic 

 axis and the line of vision is the angle beta. In emmetropia the angle alpha 

 is about 5. In hypermetropia it is greater, amounting to 7 or 8, 



