676 CAMERA OBSCURA 



shells afford the necessary variety of colour, and are soft enough to be worked with 

 facility, yet hard enough to wear for a considerable time without injury. 



The natural history of the mollusca producing the shells, and the best account of 

 the manufacture of cameos, was given by Dr. J. E. Gray, of the British Museum, in a 

 paper read before the Society of Arts in 1847, to which, and to his paper in the Phil. 

 ZVaws., we are indebted for much of the following information. 



It was the custom in Holland to use tho pearly nautilus as a cameo shell, and several 

 kinds of turbines or wreath-shells, which have an opaque white external coat over 

 nn internal pearly one. These are now rarely employed. The shells now used are 

 those of the flesh-eating univalves ( Gasteropoda ptenobranchiata zoophaga), which are 

 peculiar for being all formed of three layers of calcareous matter, each layer being 

 composed of three perpendicular laminae placed side by side ; the laminae comprising 

 the central layer, being placed at right angles with one of the inner and outer ones ; 

 the inner and outer being placed longitudinally with regard to the axis of the line of 

 the shells, while the inner laminae are placed across the axis, and concentrically with 

 the edge of the mouth of the cone of the shell. (Gray, Phil-. Trans.) 



This structure furnishes the cameo-cutter with the means of giving a particular 

 surface to his work, a good workman always putting his work on the shell in such a 

 manner, that the direction of the laminae of the central coat is longitudinal to tho axis 

 of his figure. The central layer forms the body of the bas-relief, the inner laminae 

 being tho ground, and the outer one, the third or superficial colour, which is some- 

 times used to give a varied appearance to the surface of the figure. The cameo-cutter, 

 therefore, selects for his purpose those shells which have three layers of different 

 colours, as these afford him the means of relieving his work ; and secondly, those which 

 have the three layers strongly adherent together, for if they separated, his labour 

 would be lost. 



The following are the kinds of shells now employed: 1. The bull's mouth (Cassis 

 rufum}, which has a red inner coat, or what is called a sardonyx ground. 2. The 

 black helmet (Cassis Madagascariensis), which has a blackish inner coat, or what is 

 called an onyx ground. 3. The horned helmet (Cassis cornutum), with a yellow 

 ground. 4. The queen's conch (Strombus gigas), with a pink ground. 



Tho bull's mouth and the black helmet are the best shells. The horned helmet is 

 apt to separate from tho ground, or double ; and the last, the queen's conch, has but 

 seldom the two colours marked with sufficient distinctness, and the finish of the 

 ground colour flies on exposure to light. 



The red colour of the bull's mouth extends only -a slight distance in the mouth of 

 tho shell, becoming paler as is proceeds backwards. The dark colour extends farther 

 in the black and yellow varieties. Hence the bull's mouth only affords a single cameo 

 large enough to make brooches of, and several small pieces for shirt-studs. The black 

 helmet yields on an average about five brooches, and several pieces for studs, while 

 the queen's conch affbrds only one good piece. 



Forty years since, very few cameos were made from any shells but the black 

 helmet, and the number of shells then used amounted to about 300 annually, nearly 

 all of which were sent from England, being all that were then imported. The black 

 helmet is imported into England from Jamaica, Nassau, and New Providence. They 

 are not found in Madagascar, though naturalists have for a long period called them 

 Madagascar helmets. (Gray). 



Of the bull's mouth, half are received direct from the Island of Bourbon, to which 

 place they are brought from Madagascar, and the other half are obtained from the 

 Island of Ceylon, being received by the way of Calcutta ; hence they are often called 

 1 Calcutta shells.' 



So rapidly has the trade in these shells increased, that Dr. Gray informs us, that 



in Paris 100,500 shells are used for cameos annually. These are divided as follows : 



Price Value Price Value 



Bull's mouth -. 80,000 Is. 8d. 6.400/. | Horned helmet . 600 2s. 6d. 601. 

 Black helmet . 8.000 5 1,920 j Queen's conch . 12,000 1 2J 725 



The manufacture of shell cameos was for some time confined to Italy; about twenty- 

 five years since, an Italian commenced making them in Paris, and now the trade belongs 

 to the French capital, where not less than 300 persons are engaged in the manufacture. 

 Nearly all the cameos made in France were sent to England, and in Birmingham 

 mounted as brooches, and exported to America and the British Colonies. 



CAMERA IitrciDA. An instrument invented by Dr. Wollaston, for the pur- 

 pose of enabling anyone, without a knowledge of the rules of drawing or perspective, 

 to delineate any external object with accuracy. It consists essentially of a quad- 

 rangular glass prism. 



CAMERA OBSCURA, literally dark chamber, An instrument invented by 



