60 MR. FIELD'S CHROMOMETER. 



posed between the chromascope and the spot, the aureola will 

 be by a counter refraction reduced to a spot at its centre. 



Remarks. It would be difficult to account satisfactorily for 

 the production of colors in the above experiment, by the 

 analysis of light alone, since the colored spectrum would vanish 

 if the black spot were removed. It is to be presumed, there- 

 fore, that the principle of shade in the spot concurs with the 

 principle of light in the ground in producing the colored iris. 



Experiment 2nd. If the preceding experiment be per- 

 formed with a white spot upon a black ground, in place of the 

 black spot upon the white ground, a similar spectrum will be 

 produced in which the colors will be inverted. 



It is not necessary that the objects and grounds opposed be 

 black and white to produce a spectrum ; it is sufficient that 

 they be lighter and darker with reference to each other ; nor 

 is it necessary that they be not colored, since a blue, red, and 

 yellow spot upon a ground lighter or darker than itself yields a 

 colored spectrum similar to the above, in which, notwithstand- 

 ing the particular color of the spot itself predominates, each of 

 the primary colors distinctly appears. 



Fig. V., Plate 2, represents part of the instrument invented 

 and described by Mr. Field, and called a chromometer, in which 

 A B C D is a hollow prism or wedge, in each side of which is 

 cemented and secured by a brass frame and screws, a colorless 

 plate of glass, efg h, which glasses touch each other within at 

 the end, e g, and diverge or separate at the other ends to the 

 thickness of the wedge at f h. We thus obtain a hollow 

 prism pervious to the light and vision, which might be filled 

 with a transparent colored liquid by means of an opening and 

 screw stopper in the end B D. To prevent compression of 

 the liquid the stopper should be perforated lengthwise, that the 

 air may escape and the perforation be secured by the screw or 

 plug E. 



It is evident this wedge being so charged with a colored 

 liquid, and viewed opposite the light, will throughout its broad 



