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XV. A Simple Method of Reducing Prismatic Spectra. 

 By Edwin Edser, A.R.C.'S., and 0. P. Butler, A.R.C.S* 



IN order to determine, from spectroscopic measurements, 

 the wave-lengths corresponding to the bright lines in a 

 prismatic spectrum of a metal or gas, one or other of the 

 following methods is generally used. From preliminary 

 measurements made of the deviation corresponding to a 

 number of known lines in the solar spectrum, or the line 

 spectrum of some metal or gas, a curve is drawn giving the 

 relation between deviation and wave-length. Owing to the 

 necessity of determining a very large number of points on 

 this curve in order to render its form trustworthy, this opera- 

 tion is a very tedious one, and to an observer insufficiently 

 acquainted with the reference spectrum involves great dif- 

 ficulty and uncertainty. This curve, however, having been 

 drawn, the wave-length of any line in another spectrum ob- 

 tained with the same spectrometer (no alteration of the 

 adjustments having been made) can be immediately deter- 

 mined from a measursment of its deviation. On the other 

 hand, where photographs of spectra are employed the most 

 usual practice at present is to photograph a reference solar- 

 spectrum alongside the one under examination. To an 

 observer of sufficient experience it is possible to identify any 

 of the numerous Fraunhofer lines with the corresponding- 

 lines in a Rowland's standard map ; and thus the wave-length 

 of any line in the unknown spectrum may be determined by 

 inspection. In spite of the perfection attainable by the above 

 methods when employed by a trained observer, it has appeared 

 to us that a simpler one, capable of giving accurate results 

 in the hands of an experimenter without special experience 

 in spectroscopy, might often be found of some value. The 

 production of interference-bands in a continuous spectrum 

 seemed capable of furnishing a reference spectrum which 

 could be advantageously employed for this purpose, most of 

 the difficulties incident to the above-mentioned methods being 

 entirely eliminated. We have, therefore, devoted some time 

 to the examination of various methods by which such inter- 

 ference-bands might be produced, with the object of selecting 

 the simplest, and determining the degree of accuracy finally 

 attainable by its employment. The results of our work in 

 this direction we beg to lay before the Society this evening. 



If the image of a system of rectilinear interference-fringes 



be formed in the plane of, and parallel to, the collimator slit 



of a spectrometer so that only a small part of the breadth of 



* Communicated by the Physical Society : read May 27, 1898. 



Q2 



