36 Dr. Brewster's experiments on the 
a thin film of the common yellow and white soap has the 
same property. 
20. Human hair. The fine transparent hair of a child depola- 
rises light, and possesses the most perfect neutral axes. These 
axes are parallel and perpendicular to the axis of the hair. 
21. Bristles of a sow possess the same properties as hair. 
The neutral axes are seen more distinctly in this than in the 
preceding experiment, on account of the greater magnitude 
of the bristles. 
22. The fibres of silk and wool depolarise light, and have 
neutral axes parallel and perpendicular to the axes of the 
fibres. 
23. The silkworm gut and sheep gut depolarise light. In the 
former the neutral axes are perfectly developed, but in the 
latter there is merely an approximation to them. 
24. The human cuticle depolarises light in every position. 
25. Parchment depolarises light in every direction. In two 
positions, at right angles to each other, the restored image is 
indistinct and principally nebulous ; while in other two posi- 
tions at right angles to each other, at angles of 45 0 with the 
former, the restored image is distinct. 
2 6. The horny excrescence on the human foot depolarises 
light in every position. 
27. The transparent film at the joints of the claws of the 
common Partan depolarises light; and has neutral axes pa- 
rallel and perpendicular to the length of the claw. 
28. The human nail depolarises light in every position. 
29. A quill , and the thin film which lines the inside of it, 
depolarise light. The former exhibits coloured rings, and 
the latter has distinct neutral axes. 
