mis 109 



Viteeous Humor 



The soft jelly] ike mass which fills the entire cavity of the eye behind 

 the crystalline lens is called the vitreous humor. About 99 per cent of 

 its composition is water. Its index of refraction, readily obtainable 

 with an Abbe refracto meter, is 1.336. It is a transparent, rather del- 

 icate form of very loose gelatinous connective tissue whose scanty fibers 

 are recognized only with the greatest difficulty. Occasionally a few 

 large cells have been found in it, and small rounded cells somewhat 

 resembling leucocytes are also observed in very limited numbers. These 

 various cells may cast shadows upon the retina within the visual field. 

 Such shadows possess a sort of " flitting " motion when the eyes are 

 moved while looking at a bright light. Frequently one may observe 

 them while looking through a microscope. In advanced age, crystals 

 may form in the vitreous humor, which are observed to settle to the 

 bottom of the eye when the eye is held still. 



Iris 



The eye possesses a diaphragm known as the iris. On looking into 

 an eye, one sees the pupil, which is the image of the iris formed by the 

 interposed cornea and the aqueous humor. From a geometrical optics 

 point of view the iris plays the part of an adjustable stop, through 

 whose aperture the amount of light admitted to the retina is controlled. 



The aperture of this stop is controlled by two bands of muscular 

 tissue; the sphincter muscle and the radial muscle. The sphincter 

 muscle forms a circular band under the inner rim of the iris, and its 

 contraction causes the opening in the iris to decrease in size. The con- 

 traction of the radial muscle, stretching from the rim to the outer cir- 

 cumference of the iris, causes the opening to increase in size. 



Most optical instruments containing lens systems are provided with 

 some means of blocking out such portions of a bundle of rays as are 

 undesirable for one reason or another. This blocking is usually accom- 

 plished by interposing in the path of the rays a plane opaque screen set 

 at right angles to the axis, which contains a circular aperture with its 

 center on the axis. The iris is such a perforated screen; it serves as an 

 interior stop or aperture-stop, since it lies within the system. It is 

 placed so as to decrease spherical aberration and thus " sharpen the 

 focus " of the eye and also to control the brightness of the retinal image, 

 which also depends on the size of the stop. 



If a rather powerful biconvex reading glass is used as a simple micro- 

 scope, to examine a sheet of cross-section paper, it will be seen that the 

 central portions of the image and the corners are not focused with equal 



