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169 



ot" the oyi', tVttin ;iii optic;!! point of view, is to fonii :i siuall, 

 clearly cletiiUHl, but inverted image of external objects on 

 the expansion of the oi)tic nerv(>. We will now proceed to 

 see in what manner it does this. This leads us, tirst.to a 

 brief description of tlic ucnt ral anatomy of the eye. 



As the human eye has to be moved in various directions, 

 indei)endent of the movements of the head, it must be more 

 or less globular in shape. It is thus allowed to lit more 

 easilv in the bony orbit in which it is encasetl, and the six 

 muscles which move it can more readily control its motions. 

 The ball of the eye is a hollow globe containing the lenses 

 neeessary for producing images of external objects and the 

 expansion of the optic nerve on which these images are to 

 fall. 



There are certain physical conditions which have to be 

 fulfilled in order that the image produced by any optical 

 apparatus shall be clear and distinct. We will take the 

 ])liotographer's camera as an illu.stration. In the first place 

 we find that the whole apparatus is encased in a box for the 

 protection of the more essential parts. We observe further 

 that this box is lined with black in order that no light shall 

 be reflected from its sides that will mar the distinctness of 

 the image formed by the lens. The lens is placed in front, 

 and at the back there is a screen of ground-glass on which 

 the image formed by the lens f(dls. When sitting for your 

 I)hotogra[)h you will also have noticed that the operator was 

 at great pains to properly focus the instrument so that the 

 image should be clearly formed on the ground-gIa.ss plate at 

 the back, and if you take the pains to examine his instru- 

 ment you will find just l)ehind the lens a black diaphragm 

 with a circular hole much smaller than the lens. This lat- 

 ter is for the purpose of cutting off those rays which pass 

 through the parts of the lens near the edge. These rays, 

 unless the lens is ground in the most accurate manner, add 

 nothing to the di.stinctne.<s of the image, l»ut, on the con- 

 trary, detract from its clearness of outline. 



All these conditions are carefully fulfilled in the eye, as 

 vou will see by reference to the accompanying diagram. 



