100 PRACTICAL PHYSIOLOGY 



causes a much greater convergence of the rays, and it will be possible 

 to obtain an image of the arrow upon the frosted glass screen, when 

 this is placed about three inches from the hinder end of the box. This 

 image may be easily seen on looking obliquely through the glass end, or 

 may be projected by a convex lens on a lantern screen sufficiently clear 

 for a number of observers to see. 



3. The Action of the Iris. The iris improves the definition of the 

 image by cutting out the more circumferential rays which in consequence 

 of spherical aberration would not be focussed in the same plane as the 

 more central. If the opaque screen having a central hole about an 

 inch and a half in diameter be placed in front of the convex lens the 

 total amount of light passing behind the lens is decreased, but the 

 image is now much more sharply defined. 



4. The Position of the Image. It will be noticed that if the illuminated 

 arrow point upwards the image on the artificial retina will point down- 

 wards. Images on the retina are therefore always inverted, the lower 

 half of the retina corresponding to the upper half of the field of vision 

 and conversely. By experience we always refer images on the retina to 

 their proper position in the field of vision. This rectification corre- 

 sponds to what is done by the second convex lens in projecting the 

 retinal image upon the lantern screen. The effect of this second lens is 

 to re-invert the image, so that on the lantern screen the image appears 

 in the same position as in the original object. 



5. Accommodation. It is not possible with the artificial eye to 

 mimic the changes that occur in the lens on accommodation. A clear 

 image of objects at different distances can only be obtained by shifting 

 the artificial retina backwards or forwards. 



ACCOMMODATION. 



1. The eye is able to see objects at varying distances from the 

 eye. It has the power of adapting itself so as to form a clear image 

 on the retina of different objects. Unless the eye had this power 

 images of external objects at different distances would not always be 

 formed at a constant distance behind the crystalline lens, where the 

 retina is situated. 



EXPERIMENT. Standing about 15 feet from a window and looking 

 towards it, hold up a needle about two feet from the eye. If the 

 needle be seen clearly the window sashes will be blurred, since the 

 image of these will be in front of the retina. If the window sashes be 

 looked at and seen clearly then the needle will be blurred, since the 

 image of this is behind the retina. 



2. Range of Accommodation. Determination of Near and Far 



