37 
depolarisation of light, &c. 
30. The cartilaginous breast bo?ie of a chicken depolarises light, 
and has neutral axes parallel and perpendicular to the longi- 
tudinal direction of the bone. 
31. The transparent cartilage from the shoulder of a sheep 
has neutral axes, and produces the coloured rings by polarised 
light. 
32. The transparent edge of the small fibres which compose 
the feathery part of a quill depolarise light, and have their 
neutral axes parallel and perpendicular to the axis of the 
fibre. It forms also an extraordinary prismatic image of the 
candle by transmitted light. 
33. The down of goose and ostrich feathers depolarise light. 
Feathers from the neck and tail of a cock have neutral axes 
parallel and perpendicular to the direction of the fibres. 
34. Flat bones of a cod. These bones depolarise light in every 
position, and exhibit coloured rings by polarised light. The 
soft cartilaginous substance, which is sometimes connected 
with them, possesses the same properties. 
35. Cylindrical bones of fish. These bones depolarise light, 
and have their neutral axes parallel and perpendicular to the 
axis of the cylinder. They exhibit also colours by polarised 
light. 
36. Ivory. A very thin film of ivory possesses neutral and 
depolarising axes, as perfectly as the most regularly crystal- 
lized mineral. When the vanished image is restored by the 
ivory, the intensity of the light of the other image is very 
much diminished, the difference between the two images being 
greater than I have found it in any other body. The film of 
ivory forms by transmitted light two highly coloured images 
