in the Optical Phenomena of Mother of Pearl, 405 
side of the common image, having the same angles of aber- 
ration as those seen by reflection, and resembling them in 
every respect, the blue-light being nearest the co rimon image, 
and the red light farthest from it. These two images, how- 
ever, are usually fainter thaa those seen by reflection, and 
when the second extraordinary reflected image is extinguished 
by removing the polish, it is then the most brilliant when seen 
by transmission, and in general the image which is brightest 
by reflection is faintest by transmission. 
In some irregularly formed pieces of mother of pearl which 
are ground very thin, and in which the axes of extraordinary 
reflection for the two surfaces are not coincident, four coloured 
images are seen by transmission. Two of them are produced 
by each surface, and the line which joins the two images 
formed by the same surface, always coincides with its axis of 
extraordinary reflection. It is also deserving of notice, that 
the transmitted extraordinary ray is bent towards the same 
pole as the extraordinary reflected ray to which it belongs. 
Like all other bodies, mother of pearl polarises light by 
reflection, and the angle at which the quantity of polarised 
light is a maximum, is about 59°. The two extraordinary 
images are also polarised at the same angle, but the mass of 
green and crimson light exhibits no marks of polarity. This 
substance has also the property of depolarising light in every 
position like horn, tortoise shell, caoutchouc, and gum Arabic; 
and it exhibits by polarised light those brilliantly coloured 
rings, which I have described in a former paper, as produced 
by topaz and other bodies. 
3 ^ 
MDCCCXIV. 
