Mr. Talbot's Facts relating to Optical Science. 403 



nate a great number of times with two complementary colours. 

 This a}ipeared to me so remarkable that I repeated the ex- 

 periment with additional care. The radiant point of solar 

 light was made smaller, by transmitting the ray through a 

 lens of short focus, and the position of the combined prisms 

 was slowly altered by turning them round on their centre. 

 The appearance of the bands on the paper was all the time 

 carefully noted. I soon found a position of the prisms in which 

 the remarkable phsenomenon occurred of a complete compen- 

 sation of colour: that is to say, that the bands were black and 

 white. At the same time they were become exceedingly nar- 

 row and numerous. A friend, who had the kindness to"^count 

 the lines, found one hundred and ten of them in the space of 

 two inches. On another occasion they were evidently much 

 closer, so that we estimated their number at ttvo hundred in 

 the same space of two inches. The aid of a lens was requisite 

 to see them distinctly. They resembled more than anything 

 else the closely-ruled parallel lines by which shadows are pro- 

 duced in some kinds of engraving, and which are often em- 

 ployed in maps to represent the sea. 



Now, it requires in ordinary circumstances the employ- 

 ment of verij homogeneo2is light, in order to produce bands 

 anything like these in number and distinctness. In the pre- 

 sent instance, on the contrary, common solar light was em- 

 ployed. The result therefore is quite unexpected, and it will 

 be interesting to learn in what manner it is explained by 

 theory. 



These bands are best seen in the light refected from the 

 face A C. And since the reflected ray does not enter the 

 prism ADC at all, it cannot matter, I think, of what kind 

 of glass it is composed. With respect to the other prism, 

 it appeared to me that the experiment succeeded equally well 

 whether it were of crown or of flint glass. 



§ 2. Experi^nents on Diffraction. 



In the original experiments of Grimaldi and Newton the 

 diflracted images of objects were merely received on screens of 

 white paper, by which method a great part of their brightness 

 was necessarily lost. Fraunhofer first introduced the use of 

 the telescope in these observations ; and Fresnel, I believe, 

 that of the lens or microscope. Both these were very great 

 improvements, though of an opposite character, and have 

 caused the discovery of numerous most curious ph^enomena. 



In order to see these appearances in their perfection, it is 

 requisite to have a dark chamber and a radiant point of in- 

 tense solar light, which, for the sake of convenience, should 

 3 B2 



