THE HOUSE THAT THE OYSTER BUILT. 89 



of mother-of-pearl, whenever apparent, does not depend 

 on its chemical nature, but resides on it? surface. In the 

 examination of this substance with high magnifying powers, 

 he perceived a grooved structure, closely resembling the 

 delicate structure of the skin at the top of an infant's 

 finger, or the minute corrugations often observable on 

 surfaces which are covered w r ith oil-paint or varnish. The 

 wax, which exhibited the same colours, presented also a 

 similar assemblage of grooves. Thus, as Sir Isaac Newton 

 has shown that a series of grooves breaks up a beam of 

 light that falls upon them into a number of separate parts, 

 each of which is reflected to the eye frofh the bottom and 

 sides of the little grooves, and assumes a particular colour 

 according to the angle at which it is reflected, the cause of 

 the iridescence was placed beyond all doubt. Subsequent 

 experiments proved that this singularly beautiful appear- 

 ance can be transferred to balsam of tolu, or to gum- 

 arabic, by allowing a thin film to be pressed and to solidify 

 between two pieces of mother-of-pearl ; or it may be com- 

 municated by hammering to a clean surface of lead, or to 

 the well-known compound of mercury and bismuth, which 

 forms a fusible metal. Different specimens of shell give, 

 of course, very different results as to the fineness and num- 

 ber of the grooves. Sometimes a magnifying power of six 

 or eight times will render them readily perceptible : in some 

 instances two thousand grooves have been counted in the 

 space of an inch, while in others their number was wholly 

 incalculable. It is very remarkable that grinding will not 

 obliterate these grooves. It might naturally be supposed 

 that, as the grooves must be separated by slight ridges, 

 those ridges would be worn down in the process of grind- 

 ing. But as the ridges wear down, so also do the grooves ; 

 so that, however thin the film may be rendered by grind- 



