LABORATORY EXERCISES 419 



Page 341. The muscles which move the eye can be well shown 

 by use of the eye of a dog-fish. The skull is cartilaginous and easily cut 

 away, and the muscles are diagrammatically plain. The nerves control- 

 ling them can also be identified if sufficient care is taken. 



Page 343. The eye of an ox can be easily dissected to show practi- 

 cally all the parts mentioned here. In taking off the choroid coat the 

 pigment layer of the retina usually comes away also, but the cell layers 

 will be left in position. Material should be perfectly fresh. 



Page 344. Let one student face a bright light while keeping one eye 

 covered with the hand ; others should notice difference in size of pupils 

 when the hand is removed. This makes clear the function of the iris. 



Page 348. 1. A very convincing way of showing the image on the retina 

 to be inverted is to mount the perfectly fresh eye of a chloroformed albino 

 rabbit in one end of a stiff paper tube, the cornea looking outward. A 

 person looking through the tube at the back of the eye sees the inverted 

 image of whatever is in front of it. Always look at something brightly 

 illuminated, e.g. anything out of doors on a sunny day, or an electric 

 bulb, or candle flame. 



2. A great many of the principles involved in the passage of light into 

 the eye can be well shown by use of an artificial eye apparatus. The 

 ordinary defects in the eye, as well as their correction, can also be shown 

 with it. Ordinary loose lenses can also be used as shown in the diagrams ; 

 the source of the light should be confined in a dark box, with a single hole 

 in its side, through which light rays can emerge and strike the lenses. 

 A darkened room is imperative for all experimental work with light. 



Permanent models of lenses and rays of light can easily be made. A 

 lens of any shape, regular or deformed, can be made of modeling wax. 

 This can be supported on brass rods above the middle of a board base. 

 The source of light can be represented by a point on an upright piece of 

 wood a foot or two from the lens, and the retina by a flat piece of board a 

 foot or two from the other side of the lens. String or thread can be used 

 as rays of light, and strung from source of light to lens, where they can be 

 fastened with pins as the wax is soft. The exit of rays on the other side 

 can be set up in a similar way, and their distribution on the retina shown 

 by fastening the " rays " with tacks. 



Page 351. Adaptation of the eyes to different distances is easily 

 felt by the pupil if he holds up a finger of one hand about a foot from his 

 eyes, with finger of other hand in same line two feet away. Give atten- 

 tion first to one and then to the other, Board diagrams of what happens 



