624 TEXT-BOOK OF PHYSIOLOGY. 



give rise to one color are brought to a 'focus at a point somewhat 

 different from those which give rise to other colors. If the eye is 

 accommodated for one set of rays, it is not for another, and the result 

 is a fringe of colors around the image. This defect in the normal 

 eye is so slight that the mind fails to take cognizance of it. That the 

 eye is incapable of simultaneously focalizing rays of widely different 

 refrangibility, as those which give rise to the blue and red colors, 

 is shown by the following experiment: The eye being directed to a 

 luminous point, a plate of cobalt-glass is placed between the light and 

 the observer close to the eye. This substance has the property of 

 intercepting all rays but the red and the blue and hence these alone 

 will be seen. The center of the image produced will be red and clearly 

 denned, the periphery blue and ill denned. 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 denned. 

 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 exact 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 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. 300), 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 led 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 relations of these lines and certain angles connected 

 with them are shown in Fig. 300. 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 included between the optic axis and the line of fixation or 



