424 OPTICAL INSTRUMENTS. 



chamber containing a limpid liquid called the aqueous hu- 

 mor ; the posterior chamber containing a transparent jelly, 

 V, called the vitreous humor. The vitreous humor is 

 enclosed in a transparent sack, H, called the hyaloid mem- 

 brane. The cornea, aqueous humor, crystalline lens and 

 vitreous humor are refracting media. Back of the hyaloid 

 membrane is the retina, 'R, an expansion of the optic 

 nerve. Between the retina and the sclerotic coat is N, 

 the choroid coat, intensely black and opaque. The eye, 

 optically considered, is simply an arrangement for pro- 

 jecting inverted real images of visible objects upon a 

 screen made of nerve filaments. The image thus formed 



is the origin of the sensation 

 of vision. If this image be 

 well defined and sufficiently 

 luminous the vision is dis- 

 tinct. 



658. Magnifyiiig- 

 Glasses. A magnifying- 

 glass, or simple microscope, is a 

 convex lens, generally double- 

 convex. The object is placed 

 between the lens and its prin- 

 cipal focus. The image is vir- 

 tual, erect and magnified (Fig- 

 312). The visual angle sub- 

 tended by the image is greater 

 than that subtended by the 

 object ( 587). 



FIG328 659. Compound Mi- 



croscope. The compound microscope consists of two 



