3 74 The Commercial Products of the Sea. 



the ground surface. On one side of this imperfect image 

 will be seen a brighter image, glowing with the prismatic 

 colours. On the outside of the prismatic image will be 

 observed a mass of coloured light, nearly at the same 

 distance beyond the prismatic image that the latter is from 

 the common image. These three images are always in the 

 same right line, but their distances from one another vary 

 according to the direction in which they are viewed." 



Now, it was in making certain observations on the 

 distances of these images from one another that Sir David 

 Brewster lighted upon the cause of them. He had occasion 

 to fix a piece of mother-of-pearl to a goniometer (an 

 instrument for measuring angles), by a cement of rosin and 

 beeswax. Upon removing it from the cement when in a 

 hard state, by insinuating the edge of a knife and making 

 it spring off, the plate of mother-of-pearl left a clean im- 

 pression of its own surface ; and he was surprised to 

 observe that the cement had actually received the property 

 of producing the colours which were exhibited by the 

 mother-of-pearl. This unexpected phenomenon was at 

 first attributed by him, and by several gentlemen who saw 

 the experiment, to a very thin film of mother-of-pearl 

 detached from the plate and left upon the cement ; but 

 subsequent experiments convinced him that this was a 

 mistaken opinion, and that the mother-of-pearl really 

 communicated to the cement the properties which it pos- 

 sessed. 



This circumstance sufficiently proved to Sir D. Brewster 

 that the cause, whatever it might be, of the colours of 

 mother-of-pearl resided on the surface, and did not depend 

 upon the chemical nature of the substance. In order, 

 therefore, to discover what was the configuration of the 

 surface, he applied a microscope with high magnifying 



