PRELIMINARY DIOPTRIC CONSIDERATIONS. 823 



blood) in the blood of the dog. It takes place more rapidly if the aqueous humor 

 is previously removed from the chamber through a corneal wound. Ehrlich used 

 fluorescein for the study of the movements of the fluids within the eye. This is an 

 innocuous substance which, when introduced into the body, penetrates into the 

 fluids of the eye, and may be recognized by its greenish fluorescence in reflected 

 light , even in a solution of i part to two million parts of water. From observations 

 on the entrance of this substance into the aqueous humor, it is now assumed 

 that the ciliary body is the secreting organ for the aqueous humor, which passes 

 through the pupil into the anterior chamber. 



Section of the cervical symphathetic, and still more, that of the trigeminus, 

 accelerates the secretion of the aqueous humor, but decreases its amount. 



The cornea permits the entrance of fluids into the anterior chamber, for 

 example atropin and fluorescein. 



The excretion of the aqueous humor takes place by filtration in the angleTof 

 the anterior chamber; it passes through the clefts of the spaces of Fon tana, which 

 communicate with the anterior chamber, and enters the vessels of the circular canal 

 of Schlemm, which lies directly on their outer side (plexus ciliaris venosus in 

 animals). None passes through the cornea, although some is imbibed by its 

 posterior layers, which are thus nourished, and there are no special lymph-vessels 

 to remove it from the anterior chamber. 



Under normal circumstances, the pressure is the same in the vitreous as in 

 the aqueous, although atropin seems to increase the pressure in the former, and 

 to decrease it in the latter, while physostigma has the opposite action. Arrest of 

 the outflow of the venous blood often increases the pressure in the vitreous, and 

 decreases that in the aqueous. Compression of the eyeball from without will 

 cause more fluid to pass out of the eye temporarily than enters it. The decrease 

 of the intraocular pressure after section of the trigeminus is striking; likewise its 

 increase upon irritation of the same nerve facts often observed by Landois. 

 The statements with regard to a possible analogous action of the sympathetic vary. 

 Interruption of the outflow of venous blood raises the pressure; an insufficient 

 supply, associated with a normal outflow, decreases the pressure. The innervation 

 of the vessels of the eyeball is discussed on p. 684. 



PRELIMINARY DIOPTRIC CONSIDERATIONS. 



The eye is comparable as an optical apparatus to the camera obscura. In 

 both a diminished, inverted image of objects of the outer world is found on the 

 background (the projection-surface). Instead of the simple lens of the camera, 

 however, the eye possesses several refractive media, behind one another: cornea, 

 aqueous humor, lens (the several parts of which: capsule, cortex and nucleus, also 

 possess different refractive indices) , and vitreous body. Each medium is separated 

 from the one next to it by a ref acting surface, which is assumed to be spherical. 

 The projection-surface of the eye is the retina, which is colored by the visual 

 purple. As this substance is bleached chemically by light, so that the images 

 may even be fixed temporarily on the retina, the comparison of the eye to the 

 camera is still more striking. 



In order that the passage of the light-rays through the media of the eye may 

 be accurately followed, the following factors must be known: (i) The refractive 

 indices of the media; (2) the shape of the refracting surfaces; (3) the distance 

 of the various media from each other and from the projection-surface. 



The action of a convex lens will first be considered. There are to be distin- 

 guished in such a lens the centers of curvature, that is, the centers of the 

 two spherical surfaces (Fig. 272 I, m m^. The line connecting these points is 

 called the principal axis: the center of this line is the optical center of the lens 

 (O). All rays that pass through the optical center of the lens (and which may be 

 countless) pass through unbent. They are called principal rays, or secondary 

 axes (nj). The following laws governing the refraction of rays by convex lenses 

 must be remembered: 



i. Rays falling on the lens parallel to the principal axis are so refracted that 

 they meet on the opposite side of the lens at a point that is known as the focus 

 or principal focus (f). The distance of this point from the optical center of the 

 lens (O) is called the focal distance (f O) of the lens. The converse of this propo- 

 sition is evident: Rays that diverge from the principal focus, and strike the 

 lens, become parallel to the principal axis on the opposite side, and do not meet. 



