SENSE ORGANS. 



I07 



The distance of this point from the retina is about six 

 tenths of an inch. Now when we remember that the 

 relative size of the object and image is exactly propor- 

 tioned to their relative distances from the nodal point, 

 we at once see how extremely minute the retinal images 

 of objects must be. 



COMPARISON OF THE EYE AND CAMERA. 



The purely instrumental character of the eye and its 

 mechanical perfection may be clearly brought out by a 

 comparison with the photographic camera. Take, then, 

 the dead eye and the dead camera — i. e., with only the 

 ground-glass plate in place. They are both optic instru- 

 ments for making an image. Look in at the back of the 1 

 camera and see the inverted image on the ground-glass 

 plate. Look in at the back of the eyeball and see also 

 the inverted images. Both are dark chambers, with a 

 lens in front to admit the light and make an image by 

 refraction ; both are lined within with black pigment, to 

 absorb the light and prevent reflection from side to side, 

 and so the spoiling of the image. But it is not every 

 lens that will make a perfect image. There are certain 

 defects in common lenses which must be cor- 

 rected to produce the best effects. These are 

 corrected in the best cameras and in all good 

 eyes. 



1. Chromatism. — In all simple lenses we 

 find that the images are bordered with colored 

 fringes— blue or orange. These mar the sharp- FlG ?s 

 ness of the image. This defect is corrected in 

 optic instruments by a combination of lenses of different 

 curvatures and different refractive and dispersive pow- 

 ers. In a fine telescope, for instance, a double convex 

 lens is combined with a plano-concave lens. The one is 

 crown glass, the other flint glass (Fig. 75). Such a corn- 



