14 FIELD'S CONVEX AND CONCAVE PRISMS. 



There are no two colors by which we can form any of the 

 three primary colors, while we can produce all shades of purple 

 and violet from red and blue, all shades of green from blue and 

 yellow, all shades of orange and gold colors from yellow and 

 red, and from the three primary rays, in given proportion, 



BLACK. 



But if light is transmitted through a piano-conical prismatic 

 lens, the angle of which is 23 degrees, it invariably exhibits in 

 annular form, well defined, the three original colored rays of 

 which light is composed. See Fig. 1. Plate II. 



Mr. Field observes, page 217 of his ^Esthetics, or Analogy 

 of Sensible Sciences " In the centre of a piece of white paper 

 six inches square, form a black spot (one-tenth of an inch in 

 diameter), place it upon the stand of a chromascope, opposite to 

 the light of a window, and adjust the instrument in its vertical 

 position, so that the spot may be close to and concentrical with 

 \helenticular prism, or double convex prismatic lens ; then gra- 

 dually sliding the tube upward, looking at the same time 

 through it, the spot will appear to expand and be refracted 

 into a beautiful annular colored spectrum, or aureola. When 

 a black spot is made on a white or light ground', the rotation of 

 the colors of the iris is blue, red, and yellow. See No. 2, 

 Plate I. 



If, however, you place a white spot on a black or dark 

 ground, the position of the colors is inverted, being red, 

 yellow, and blue. See No. 3, Plate I. 



If now the double concave lenticular prism, Fig. 2, Plate III., 

 be interposed between the chromascope and the spot, the 

 aureola will be, by a counter refraction, reduced to the spot at 

 its centre. 



From the action of the double convex prismatic lens of Mr. 

 Field on the refraction of the dark or black spot, it would 

 seem that the shade is composed of the rays of light in such 

 form of combination as to be black, analogous with carbon, as I 

 am satisfied it will yet be demonstrated that carbon is a 



