376 The Commercial Products of the Sea. 



culable. What is very remarkable is that grinding will not 

 obliterate these grooves. It might be supposed that as the 

 grooves must be separated from one another by slight 

 ridges, these ridges might be worn away in the process of 

 grinding. But as the ridges wear down, so do the grooves 

 also ; so that, however thin the film may be rendered by 

 grinding, the grooves and the colours resulting from them 

 are still developed. If the surface has any scratches or 

 dents, the bottoms and sides of the scratches are grooved, 

 just as if the surface had been level. 



If we view a candle through a thin film of mother-of- 

 pearl, or of gum or balsam which has received the grooved 

 impression, coloured images of the candle will be seen 

 nearly as distinctly as when the light is reflected from its 

 surface. 



If a scientific statement be true, there are generally 

 means for proving its applicability in more circumstances 

 than one. Consequently, if the colours of mother-of-pearl 

 are produced by grooves on its surface, any mechanical 

 contrivance by which similar grooves may be produced on 

 any substance ought to give similar results. This has been 

 strikingly confirmed by Mr. Barton, of the Royal Mint. 

 This gentleman has constructed an engine by which he can 

 engrave on the surface of steel and other metals lines so 

 exquisitely minute that from 2000 to 10,000 are included 

 in a single inch. These surfaces, when viewed by daylight, 

 present but few appearances of colour ; but when the light 

 of the sun or of gas flames falls upon them, an extremely 

 brilliant display of colours is the result ; every gradation 

 of tint is exhibited, and a change is produced by every 

 motion of the object or of the source of illumination. 



There are six or eight leading varieties of mother-of- 

 pearl shells entering into commerce. 



