, 



APPLICATION OF FIELD S PRISMS. 61 



face present a perfect gradation of color from the utmost dilute- 

 ness or minimum, at the convergent extreme, g e, where the 

 glasses touch, to the utmost depth or maximum, at the divergent 

 extreme fh, where they are at their utmost separation. 



On one side of the wedge C D is screwed a brass scale of 

 the exact length of the cavity within the glasses e f g h, 

 geometrically divided into 32 degrees, each subdivided into 4 

 others, forming evidently an accurate measure of thickness 

 increasing at each division from the point of contact of the 

 glasses, and are consequently also a true numerical measure of 

 the intensities of transparent color without the wedge. Such a 

 prism, charged with a blue liquid, will form a cyanometer, or 

 measure of blue ; with a red liquid, an eruthrometer, or measure 

 of red ; and with a yellow liquid, a xanthometer, or measure of 

 yellow, the colors of such liquids being adjusted to a given 

 intensity.* 



By the application of the convex lenticular prism, Mr. Field 

 has effected, 1st, the refraction of a beam of perfect white light 

 into the three primary colored rays of red, yellow, and blue, 

 perfectly distinct, and forming a circular iris without the 

 appearance or intervention of any of the heterogeneous or 

 compound rays which are usually occasioned by the obliquity 

 and superposition of the rays exhibited in the imperfect spec- 

 trum produced by the Newtonian or common triangular prism. 



2ndly. That these original colored rays, the red, yellow, and 

 blue, are immediately restored to reunion, and form pure white 

 light by the simple intervention of the double concave lenticular 

 prism, Fig. 2, Plate II., by which the identity of these three 

 colors in the formation of light is doubly proved ; first, by 

 analysis, and ultimately by synthesis. 



* This description of the instruments invented and applied by Mr. Geo. 

 Field is taken from his'-jEsthetics, or The Analogy of the Sensible Sciences," 

 and also from his " Chromatography, or Treatise on Colours and Pigments," 

 (published by Tilt, Fleet Street,) to which for other most interesting and valu- 

 able information the reader is referred. 



