154 ELEMENTARY BIOLOGY. [CHAP. 



and then twenty-four hours or more in alcohol. 

 Examine its surface with one inch obj. with re- 

 flected light. It will be seen to be marked out 

 into a great number of minute square areas 

 or facets, each of which shews faint signs of 

 furrows crossing it diagonally from corner to 

 corner. 



b. Imbed the eye and cut a number of sections from 

 it perpendicular to its surface : mount in glycerine 

 and examine with one inch objective. 



a. If the section has passed through the middle 

 of the eye it will be seen to present a central 

 mass (optic ganglion) from which a number of 

 lines appear to radiate to the facets on the 

 surface. These radiating lines (which are 

 obscured here and there by concentric pig- 

 mented layers) are indications of the striated 

 spindles, connective rods and crystalline cones. 



c. Examine your thinnest section with a high power, 

 or tease out one of your thicker ones in gly- 

 cerine. Beginning at the exterior make out suc- 

 cessively 



a. The cornea, answering to one of the superficial 

 facets. Its flat outer and slightly convex inner 

 surface. Immediately beneath the cornea 

 there will be seen (in good specimens) a 

 slightly granular layer. 



ft. The crystalline cone, an angular transparent 

 body which is usually obscured by pigment. 

 If this is the case, another section must be 

 mounted in dilute caustic potash, which re- 

 moves the pigment. 



