124 DIFFRACTION. 



equal parts, the areas of the two portions of the curve being 

 equal. 



The law of the intensity in the diffracted spectrum was 

 deduced by Mossotti from the foregoing : it is represented by 

 the ordinates of the curve in the following diagram. We see 

 that 



I. The ordinate which divides the light into two equal 

 portions corresponds to the middle point of the spectrum. 



II. This ordinate is a maximum ; and the curve is sym- 

 metrical with respect to it as an axis. 



Accordingly, the intensity of the light of the diffracted spec- 

 trum is a maximum at the middle of its length, and decreases 

 thence symmetrically on either side. It is evanescent, when 

 the wave-length increases, or decreases, by about one-third 

 of the value corresponding to the maximum intensity. Hence 

 while in the spectra formed by refraction the ratios of 

 the spaces occupied by the several colours, and the intensi- 

 ties of the light at the several points, vary with the refracting 

 substance, they are, on the other hand, invariable in the dif- 

 fracted spectrum. The latter spectrum, accordingly, must 

 be regarded as the normal one, to which all others are to be 

 referred. 



(137) Gratings producing these effects may be formed in 

 several ways as, for example, by tracing a number of paral- 

 lel lines on glass with a fine diamond point. Fraunhofer suc- 

 ceeded by such means in forming ruled surfaces in which the 

 striae were actually invisible under the most powerful micro- 

 scopes, the interval of the grooves being only the ^oiou of an 

 inch. 



Analogous phenomena may be produced by reflexion. If 



