CHEMICAL EFFECTS DESCRIBED BY WAVE-LENGTHS AND TIMES OF VIBRATION. 59 



as exciting so powerful an influence in the distribution of light and heat, are equally 

 perceptible in the case of chemical effects, a result which becomes apparent at once 

 when we operate with the interference spectrum. When, on any sensitive silver sur- 

 face, this spectrum is received, instead of a stain exceeding in length the coloured spa- 

 ces, the change is limited to a narrow region, occupied by the more refrangible rays. 



208. lojig. 133, the visible interference spectrum is given with its fixed lines, as it 

 appears on the ground glass of the camera. The figure in the frontispiece is drawn 

 from the measures of FRALNHOFER. If the two be compared together, it will be seen 

 that there are differences in the relative distances of the lines. These differences arise 

 from the mode of conducting the experiment in the two cases. FRACHHOFER'S spec- 

 trum was carefully determined, the angular deviations being measured on a graduated 

 arc. Fig. 133 is depicted from the appearance on the flat ground glass of the camera. 



209. Before giving a description of the chemical effects of this spectrum, it is neces- 

 sary to explain the method of subdividing the spectrum which is here resorted to. In 

 the earlier discussions on the chemical effects of light, the different regions of the spec- 

 trum were marked out by the designations of the different coloured rays, and effects 

 were described as taking place in the red, or yellow, or violet ray. An improved plan 

 was proposed by Sir J. HERSCHEL, and followed by him in his various writings : it 

 consists essentially in dividing the space which exists between the red and yellow ray, 

 as insulated by cobalt blue glass, into 13-30 parts, taking the centre of the yellow ray as 

 the zero point, and continuing the divisions equally into the more and less refrangible 



regions. 



210. Over these different methods the use of the fixed lines possesses very great ad- 

 vantages, inasmuch as we do not make reference to ideal, but to actually visible points 

 existing in the spectrum. Since the discovery of these lines, therefore, both M. BEC- 

 QUEREL and myself have used them to mark out spectrum regions. The only difficul- 

 ty which is in the way is, that they do not give subdivisions minute enough for many 

 purposes. But this difficulty can be wholly removed, and other very great advantages 

 gained, by using them in the manner which I shall now proced to explain. 



211. It has been stated that the deviations (189) of the different fixed lines, B, C, D, 

 in the interference spectrum, are proportional to the lengths of the undulations which 

 thev respectively represent. Bv designating the different points of the spectrum by 

 their wave-lengths, the subdivision may be carried to any degree of minuteness; the 

 measures of one author will compare with those of another, and the different phenom- 

 ena of chemical changes occurring through the agency of light become allied at once 

 with a multitude of other optical results ; as, for example, when we are told that the 

 decomposition of bromide of silver occurs at a maximum under the influence of a ray 

 which is 0-00001538 of a Paris inch in length, we recognise in an instant that this ray 

 falls between the fixed lines G and H, the length of G being 0-00001587, and of H 

 0-00001464; we see, moreover, that the point spoken of is nearer to G than to H, and, if 

 it were necessary, by a very simple arithmetical process, we could determine the number 

 of vibrations executed by that ray in thus bringing about the maximum decomposition in 

 billiontbs of a second, using the rate of the propagation of light at 192,000 miles. 



