and Coloured Shadows. 441 



edge of the object and a narrow border of the ground upon 

 which it is placed, in succession. These alternations in 

 the direction of the eye produce the accidental colour 

 which appears to play about the edge of the object, and, 

 also, by the intermissions which they occasion, preserve 

 the colour of this part of it from being impaired. If, for 

 instance, the eye be directed to a red seal upon white 

 paper, after a little time there will be observed a faint blue 

 border surrounding the seal, which will increase in inten- 

 sity as the colour of the seal declines ; and the edge of the 

 seal to a depth nearly corresponding with the breadth of 

 the blue border, will be found to preserve its original 

 strength of colour. 



When the eye is impressed with any moderately illumi- 

 nated object, such as the seal in the last experiment, the 

 complementary colour surrounding it, which is seen upon 

 the white ground, is not of sufficient intensity to produce 

 a corresponding accidental colour, and the image formed 

 by directing the eye to a different part of the ground is not 

 surrounded with any border; but when the eye is im- 

 pressed with the strong light of the sun, the accidental 

 colour, formed at the same time upon the edges of its 

 image by the white surrounding atmosphere, corresponds 

 in intensity with the original impression, and the accidental 

 colours of both are seen together. 



There is an interesting paper on Coloured Shadows, by 

 Count Rumford, published in the Philosophical Transactions 

 for 1794, and also in the 1st volume, quarto series, of Nichol- 

 son's Journal ; and a memoir on the same subject, by 

 Citizen Hassenfratz, published in the Journal de L'Eole 

 Polytechnique, tom. iv.; and a translation of it in the 6th 

 and 7th volumes, octavo series, of Nicholson's Journal. 



This paper was written when I had no opportunity of 

 consulting these works; but, upon since referring to them, 

 I do not find it necessary to make any alteration. 



In one of Count Rumford 's experiments, he prepared 

 two Argand lamps, and by intercepting their light at a 

 proper angle, by means of a flat ruler, he procured two 

 broad shadows which were projected upon white paper and 

 found to be perfectly colourless. He then directed a tube 

 about twelve inches long, and an inch in diameter, lined 



