LENTICULAR PRISM OF FIELD. 59 



generated a similar double concave lenticular prism, opposed to 

 the above, as represented in the next figure. Plate II., Fig. 2. 



The above figures are sufficient for the present purpose ; yet 

 it is evident that they are extremes between which lie an inde- 

 finite series of intermediate figures, and that there are* innu- 

 merable others, both conical and annular, generable upon the 

 boundless variety of figure and motion. 



To apply the prism thus varied to the purposes of experi- 

 mental philosophy, an instrument has been contrived by Mr. 

 Field, of which the following is a brief description, and which, 

 from its power of exhibiting colors, he has denominated the 

 chromascope. 



This instrument in its general form consists either of a 

 tripod, or of a base or stand, having two arms or props which 

 support the axis of a tube five inches long, at the lower end of 

 which is the lenticular prism above described (Fig 1). The 

 principal tube slides from end to end, through a short tube 

 attached to the axis, and has at top a narrow sliding tube of 

 three inches fitted with a lens or eye-glass of eight inches 

 focus. See Plate II., Fig. 3. 



By this construction the tubes may be elevated or depressed 

 above the stand, and at the same time turned and fixed at any 

 required angle of the horizon. 



Such are the instrument and its parts ; the design of which 

 will be rendered apparent in its application to the following 

 experiments. 



Experiment 1st. In the centre of a piece of white paper 

 six inches square form a black spot, place it upon the stand of 

 the 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 the lenticular prism 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. 



If now the double concave lenticular prism (Fig. 2) be inter- 



