OPTICS. 



39 



an impression of the mother of pearl 

 upon black wax, when very hot, or in- 

 deed upon any other cement, and ex- 

 amine the surface of the wax by looking 

 at the candle in it, we shall see all the 

 phenomena which we saw in the mother 

 of pearl, excepting the mass of crimson 

 light. If the mother of pearl is un- 

 polished, the wax will exhibit only one 

 of the prismatic images, and it will be 

 on the opposite side of the ordinary 

 image from what it was on the mother 

 of pearl ; because the impressed surface 

 must be the reverse of the impressing 

 surface. 



We may imitate artificially and more 

 perfectly the action of the mother of 

 pearl, by placing Isinglass or Gum 

 Arabic, or Balsam of Tola between 

 two surfaces of mother of pearl. When 

 the enclosed substance has dried or 

 cooled, if it is Balsam of Tolu, and is 

 removed from the mother of pearl sur- 

 faces, it will display by reflexion from 

 either of its surfaces, and also by trans- 

 mission, the fine prismatic colours of 

 the natural shell. An impression of 

 mother of pearl may also be taken upon 

 the fusible metal composed of bismuth 

 and mercury ; and by hard pressure, or 

 the blow of a hammer, it may also be 

 made upon cold lead. On the fusible 

 metal the play of the colours is singu- 

 larly fine ; but from a gradual change in 

 the crystalline state of the metal, the 

 surface soon loses its polish, and the 

 colours disappear. 



From these facts it is obvious, that 

 all the above phenomena of mother of 

 pearl, as seen by reflected and transmit- 

 ted light, have their origin in a particu- 

 lar configuration of its surface ; that 

 the communication of its properties to 

 other bodies is the necessary conse- 

 quence of the communication of its 

 superficial structure, and that none of 

 the light, excepting that which produces 

 the mass of crimson, has penetrated the 

 surface of the mother of pearl. 



In order to ascertain what this con- 

 figuration of surface was, we examined it 

 with very high magnifying powers, and 

 found that every specimen which exhi- 

 bited these colours had a grooved 

 structure, resembling the delic.ite tex- 

 ture of the skin at the top of an infant's 

 finger, or the wrinkles which are often 

 seen on surfaces covered with varnish 

 or with oil paint. We have attempted 

 to represent this surface in (Jig. 40.), a 

 small portion of which, in n, has the 

 grooves nearly parallel. In most cases, 



Fig. 40 



however, the grooves are irregular, 

 and as the line joining the coloured 

 images is always perpendicular to the 

 direction of the grooves, these irregular 

 specimens give coloured images lying in 

 almost every direction. Upon measur- 

 ing the distance of these grooves, we 

 found them to vary from 200 to upwards 

 of 5000 in an inch ; and in every case 

 the distance of any of the prismatic 

 images from the ordinary image increa- 

 ses as the grooves become smaller and 

 closer. In a specimen with 2500 in an 

 inch, the distance of the prismatic 

 image from the ordinary one is 3 41', 

 and in a specimen of 5000 in an inch, 

 the distance is 7 22', almost exactly 

 double. For every thousand more 

 grooves that there are in an inch, the 

 prismatic images separate from each 

 other half a degree. Hence in most 

 specimens of mother of pearl, when 

 both the direction and the distances of 

 the grooves are various, we have the 

 prismatic images scattered in various 

 directions, and at various distances from 

 each other. 



One of the most remarkable circum- 

 stances in mother of pearl, is that when 

 we grind down the natural surface with 

 the finest powders, and polish it to the 

 utmost degree of brilliancy, we shall 

 never be able to grind out the grooved 

 structure which produces the colours. 

 The substance of the shell disappears, 

 but the depressions as well as the eleva- 

 tions on the surface are worn away at 

 the same time. It deserves to be re- 

 marked too, as a curious circumstance, 

 that on grinding down the surface after 

 it is polished, one of the sides of the 

 grooves seems to be worn down, while 

 the other is scarcely at all affected by 

 the process. 



Mr. Herschel has discovered in mother 

 of pearl another pair of nebulous colour- 



