THE DIFFRACTION SPECTRUM. [MEMOIR VI. 



The method by which the first photograph of the dif- 

 fraction spectrum was obtained is described as follows 

 in the Philosophical Magazine, Jline, 1845. 



The prismatic spectrum, even when every precaution 

 has been used to obtain it in a state of purity, its fixed 

 lines being visible, is liable to lead us into many errors. 

 As respects its luminous or photic properties, we cannot 

 determine the distribution or intensity of the light be- 

 cause the violet extremity is unduly dilated. As respects 

 the chemical effects, the same difficulty occurs, for these 

 are necessarily controlled and disturbed by the law of 

 distribution. All chemical actions occurring in the more 

 refrangible regions, by being spread over a great space 

 appear to be more feeble than what they actually are. 



In a perfect spectrum the most luminous portion of the 

 yellow should be in the centre ; and from this the inten- 

 sity of the light should gradually decline, fading away 

 on one side in the red, and on the other in the violet. 

 At equal distances from the middle yellow point on 

 either hand the intensity of the light should be equal. 

 These beautiful results are due to Mosotti, who also 

 states that the length of the extreme red wave is to that 

 of the extreme violet in the simple ratio of 2 : 1. 



The prismatic spectrum does not exhibit these facts. 

 The yellow is not in the centre ; the blue, indigo, violet 

 are abnormally spread out, the spectrum having its own 

 law of distribution. But the diffraction spectrum en- 

 ables us to observe them. 



By the aid of a heliostat, I arranged horizontally in a 

 dark room a narrow ribbon of light, coming through a slit 

 3*0 of an inch wide, set vertically. At the distance of 

 twelve feet it fell perpendicularly on a piece of flat glass, 

 the surface of which was ruled with equidistant parallel 

 lines by a diamond ; and having been silvered with tin 

 foil, after the manner of a mirror, it served the purpose 



