in the Optical Phenomena of Mother of Peart. ggg 
Sometimes it is composed of parallel or concentric laminae: 
sometimes the veins are inflected in various successions, and 
sometimes it exhibits the same appearances as those which 
constitute what is called the hammered agate. 
Tlie regularly formed mother of pearl is of an uniform 
whiteness, somewhat resembling tlie pearl itself, and, in day 
light, scarcely exhibiting any of the prismatic colours ; and, 
unless it is expressly mentioned, this is always the kind which 
I have used in the following experiments. 
Let AB Fig. i. (PI. XIV.) be a plate of mother of pearl, not 
polished, but having its two surfaces ground perfectly either 
upon a blue hone, or upon a plate of glass with the powder of 
schistus, and let the light Rrof a candle be incident at any angle 
on the point r, this ray will be reflected according to the 
ordinary law, so that the angle RrC is equal to CrS, and the 
lines Rr, Cr and Sr in the same plane. 
If the eye is now placed very close to the mother of pearl 
at B, so as to receive the reflected rays, it will perceive at S, 
in the direction rS' the common reflected image of the candle 
which will not be very bright, owing to the roughness of the 
reflecting surface. On the lower side of S' at the distance of 
some degrees there will also be seen a highly coloured image 
of the candle at s, formed by rays reflected in the direction rs. 
In this spectrum the blue rays are nearest the common 
image, and the colour is so great, that it requires a prism of 
flint glass with a refracting angle of 65* to correct it, a large 
secondary spectrum being left, having the uncorrected green 
towards the vertex of the prism. 
If the candle at R is kept steady, and the plate AB turned 
round r as a centre, so that the ray Rr may preserve the same 
3 F 2 
