THE SENSE OF SIGHT 



703 



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

 for this is dear. 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 o 



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



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



I 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 



Temporal Jige^ 





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



of the 





^s about 5. In hypermetropia it is greater, amountmg to 7 or 8 , 



