120 DIFFRACTION. 



being held before the object-glass of the telescope, the spectra 

 are formed at its focus, and are there viewed, with all the ad- 

 vantages of distinctness and amplification, by means of the 

 eye-glass. Fraunhofer, who observed these phenomena with 

 much attention and care, traced no fewer than thirteen spec- 

 tra on either side of the central image ; the first pair being 

 separated from the central image, and from the second pair, 

 by intervals absolutely black. By a very accurate mode of 

 measurement he ascertained that the deviations of any one 

 colour from the central image, in the successive spectra, formed 

 an arithmetical progression ; and that the absolute amount 

 of these deviations varied inversely as the intervals^ of the 

 axes of the wires. 



(133) The foregoing results flow readily from the principle 

 of interference, ^hQ first pair of spectra, on either side of the 

 central image, being produced by the interference of those rays 

 whose paths differ by one undulation ; the second pair, by those 

 whose paths differ by two undulations ; and so on. 



Let the light proceeding from a very remote origin fall on 

 the grating, whose opaque parts are represented by ab, a'b', 

 a"b", &c. ; and let Q, be the place of the eye. Then, if we take 

 a portion of the grating, a' a", composed of one opaque and one 

 transparent portion, in such a manner that the difference of the 

 distances of its extremities from the point Q, Qa" - Qa', shall 

 be equal to the length of 

 a wave, it is manifest that 

 the corresponding portion 

 of the incident wave, a' a" 

 may be divided into two 

 parts very nearly equal, 

 the waves sent from which 

 to the point Q shall be 

 in complete discordance. Without the grating, therefore, the 



