il4 Dr Young's theory of the Colours discovered 



*' There are two methods by which we may calculate the 

 properties of a combination of several undulations : The one is, 

 to confine ourselves to a given instant to find the sum of the 

 motions which take place for any corpuscle, and to calculate 

 after the maximum and minimum for every possible instant. 

 The other is, to determine for every differential, or individual 

 addition of motion the properties of the whole resulting un- 

 dulations. M. Fresnel made use of the latter, which is per- 

 haps the most general and elegant ; but the first is the most 

 simple, and is applicable without difficulty to the instants of 

 maximum and minimum by the aid of the general principles 

 of variable quantities. 



" Let us suppose, for example, that there are 100 parallel 

 diffracting lines traced at the distance of the 5000dth of an 

 inch from each other, and all equally distant both from the 

 source of light, and from the card or lens which receives the 

 images, and let us consider only the homogeneous green light 

 whose undulations are nearly the 50,000dth of an inch. It 

 follows from experiments which I pubhshed in ]801, that each 

 pair of the lines will exhibit brilliant bands at angular distan- 

 ces from the middle point, whose series are the numbers ^^y 

 j^i /oj ^o !?• 1'^^ ^^"^^ which I had then at my command 

 not being equidistant, I was unable to draw from them the 

 remarkable consequence which the experiments of M. Fraun- 

 hofer have since given,* and which I had observed also on the 

 Iris buttons of Mr Barton ; that is, that each colour is con- 

 tained within limits as well marked as those of the solar spec- 

 trum produced by refraction ; whereas, with a single pair of 

 lines, or with a single narrow wire, we distinguish only con- 

 fused and mixed colours, as in the reflected rings of Newton. 



* This result was established so long ago as 1813 by Dr Brewster's ex- 

 periments on a mother-of-pearl spectrum, which he found to be capable of 

 being corrected by the opposite action of a prism of flint glass of 65° " a 

 large secondary spectrum being- left, having the uncorrected green towardi 

 the vertex of the prism." Phil. Trans. 1814. By this experiment, not 

 only was the general resemblance of the spectra established, but also their 

 specific difference, or the fact, that the least refrangible spaces were more 

 expanded, and the most refrangible ones m^ire contracted in the mother-of- 

 pearl than in the glass. — Ei> 



