28 The American Naturalist. [January, 



small mammals), the plane of the section passing through 

 each fovea on the centre of the area centralis. Fig. 4 repre- 

 sents such a section through the foveas a and b of a chickadee's 

 head (Parus atricapillus), while the lines G H and G I show 



the axis of vision. The dotted lines c mark the position of the 

 optic nerves which enter in a plane much lower down. In 

 order to harden the whole head, and, at the same time, decal- 

 cify the bone, it must remain longer in Perenyi's fluid (about 

 thirty- six hours), and to preserve the cornea and lens in posi- 

 tion, a window is made in the top of the eye that the fluids 



Having had good success with simple immersion of the 

 head, this method was tried for hardening the small eyes, and 

 with good success. In fact, the retina proved in good condi- 

 tion, if not better, than when taken through by the injection 

 method. The eye-ball, however, usually caves in when placed 

 in 70 per cent, or 80 per cent, alcohol, but this may be pre- 

 vented by simply making a small slit through the sclerotic 



