160 



INVERSION OF THE IMAGE. 



than the atmosphere, the beam will be still more con- 

 verged. But what becomes of the rays that fall on the 

 iris ? They cannot all pass through the pupil. 



Dr. B. Those which fall on the iris are reflected 

 back through the aqueous humor, and help to produce 

 the peculiar lustre of the eye. 



Emily. They have now to pass through the nearly 

 spherical crystalline lens, which being convex, will, I 

 suppose, still farther increase the convergency of the rays. 

 Dr. B. Yes ; it will act precisely like a convex 

 lens, bringing the rays sooner to a focus, in their passage 

 through the vitreous humor. On account of the lesser 

 density of the latter humor, they will lose a little of their 

 convergendy, till they at last, fall on the retina, and 

 there form the image of the object from which they pro- 

 ceed. But this image will represent the object in an 

 inverted position the cause of which, I presume, you 

 will not need my aid to explain. 



Emily. The rays which enter the pupil from the 

 upper portion of the cornea, will pass on in a downward 

 direction, and fall on the lower 

 part of the retina. Those entering 

 it from below will take an upward 

 direction, crossing the others in 

 their course, and fall on the upper 

 portion of the retina, while the 

 rays which pass through the pupil 

 perpendicularly to it, will strike 

 on the centre of the retina. O, 

 here it is illustrated in this figure. 

 Dr. B. If you clearly under- 

 stand the subject so far, you can, 

 no doubt, readily see what would 

 be the effect of an increase of the 

 convexity or density of the humors, 

 on the image of the object. 



Emily. In that case, I sup- 

 pose, the rays would be brought 



