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. 



/?. 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. 



