THE EYE AND THE SENSE OF SIGHT 121 



in one of the fluids used in hardening and preserving the brain 

 (Ex. 6). First, the six muscles of the eyeball should be noticed and 

 identified. To examine the internal structures an equatorial incision 

 should be made about halfway between the cornea and the back part 

 of the eyeball, thus dividing the eye into two parts. The structures 

 thus made visible can be identified by reference to the corresponding 

 parts of the text. If the teacher has any knowledge of histological 

 methods, some excellent sections for study with the microscope can 

 be prepared by hardening the eye of a rat or frog in Perenyi's fluid 

 for two or three days, following with alcohols of increasing strength, 

 infiltrating and embedding in celloidin, and sectioning on a micro- 

 tome. Sections made horizontally through the entire eyeball, and 

 properly stained, show not only the different coats and inclosed 

 structures in situ, but the different layers of the retina, the entrance 

 of the optic nerve, or blind spot, and the yellow spot. Similar sec- 

 tions can be purchased. 



48. Refract 'ng Media. Refraction of light should be demonstrated 

 by means of lenses of various forms. Especial attention should be 

 given to the formation of images by convex lenses. The office of 

 the lens of the eye can be shown by removing it from the eye of a 

 recently killed animal and allowing the direct rays of the sun to be 

 focused by it. 



49. Inversion of the Image on the Retina. This can be very easily 

 shown by cutting away the posterior part of the sclerotic coat of a 

 fresh ox eye, leaving the retina intact. Then on turning the cornea 

 toward some bright object, as a candle flame, an inverted image 

 of the object may be seen shining through the retina. The image 

 appears much clearer if the eye is placed, cornea forward, in a tube 

 of blackened paper. 



50. Model for demonstrating the Optical Properties of the Eye. At 

 little labor and expense the teacher or pupil can construct simple appa- 

 ratus that will illustrate many of the optical features of the eye. Pre- 

 pare an oblong box from twelve to eighteen inches long, open on one 

 side, and blackened within (Fig. 71). One end should be perforated in 

 the center by an opening one half to one inch in diameter, to represent 

 the pupil of the eye. A watch crystal can be fastened over the open- 

 ing, outside, to represent the cornea. The amount of light admitted 

 through the opening in the box can be regulated by means of paper 

 diaphragms with different-sized perforations. Inside the box a reading 



