364 P. C. on the Colours that enter into the [Nov. 



only as are necessary to explain the principles upon which 

 they are made, and applied ; leaving it to the judgment of 

 your readers to supply the deficiency, either to confirm or 

 to refute what I have advanced, as they may consider most 

 conducive to the discovery and support of truth, the great 

 objects of philosophical investigation. 



I have found it very convenient, as well as instructive, 

 in many cases, to admit light through an aperture in a card, 

 of about a tenth of an inch in breadth, and about an inch in 

 length ; before which such coloured glasses may be placed 

 as are known to transmit only certain colours ; or other 

 glasses, the absorbing power of which it maybe desirable to 

 ascertain ; in order that the light transmitted by them may 

 be analyzed by the prism. 



If we place an orange-yellow glass, of a tint just suffi- 

 ciently dark to prevent the transmission of any violet light, 

 between the window and the aperture in the card, the light 

 admitted by it will, of course, be yellow; but if a prism be 

 interposed between the aperture and the eye, the yellow 

 will Ibe divided into green and red, which, if the glass be 

 not of too dark a colour, will be nearly equal in breadth. 

 If we vary the experiment, by covering only a part of the 

 length of the aperture with the coloured glass, we shall 

 have a complete and simple analysis of white and yellow 

 light at one view. 



I have formed another class of experiments, by causing 

 the prismatic fringes from a white card to fall upon a 

 coloured ground, and thus produce either white, or some 

 other compound colour. Upon a red ground, for instance, 

 such as scarlet moreen, when the reflection from the two 

 sources reach the eye in the proper proportions, the ground 

 becomes white where the blue fringes fall, and crimson with 

 the violet fringes. 



The following simple experiment, upon this principle, is 

 very conclusive. I have a hearth rug the border of which 

 is formed of stripes of different colours, of about an inch in 

 breadth ; consisting of a black, a red, a light blue, a light 

 green, and a yellow, in the order in which they are named, 

 upon directing the prism to this border, the red retains its 

 proper colour ; the blue becomes green : and the green is 

 converted to blue ; the exchange of these colours is so com- 



