The London, Edinburgh and Dublin philosophical magazine and journal of science . cose rcos2v+Rcos2 n ~ rcos2i>+RcosaM for Optical Purposes. 203 since these values satisfy the equation. The line of foci isthen a circle with a radius equal to one half p. Hence, if asource of light exist on this circle, the reflected image and allthe spectra will be brought to a focus on the same circle. Thisis, if we attach the slit, the eyepiece, and the grating to thethree radii of the circle, however we move them, wre shallalways have some spectrum in the focus of the eyepiece. Butin some positions the lin
The London, Edinburgh and Dublin philosophical magazine and journal of science . cose rcos2v+Rcos2 n ~ rcos2i>+RcosaM for Optical Purposes. 203 since these values satisfy the equation. The line of foci isthen a circle with a radius equal to one half p. Hence, if asource of light exist on this circle, the reflected image and allthe spectra will be brought to a focus on the same circle. Thisis, if we attach the slit, the eyepiece, and the grating to thethree radii of the circle, however we move them, wre shallalways have some spectrum in the focus of the eyepiece. Butin some positions the line of foci is so oblique to the directionof the light, that only one line of the spectrum can be seenwell at any one time. The best position of the eyepiece, asfar as we consider this fact, is thus the one opposite to thegrating and at its centre of curvature. In this position theline of foci is perpendicular to the direction of the light; andwe shall show presently that the spectrum is normal at thispoint whatever the position of the slit, provided it is on the circle. Kg. Fig. 1 represents this case. A is the slit, C is the eyepiece,and B is the grating with its centre of curvature at C. Inthis case all the conditions are satisfied by fixing the gratingand eyepiece to the bar B 0, whose ends rest on carriagesmoving on the rails A B and A C at right angles to eachother. When desired, the radius A D may be put in to holdevery thing steady; but this has been found practically unne-cessary. The proper formulae for this case are as follows. If X isthe wave-length, and w the distance apart of the lines of thegrating from centre to centre, then we have1_XNC~2towhere N is the order of the spectrum; sm2 NNow in the given case p is constant, and so NX. is propor- 204 Prof. H. A. Rowland on Concave Gratings tional to the line A C. Or, for any given spectrum, the wave-length is proportional to that line. If a micrometer is fixed at C, we can consider the case asfollows: 1 JtN . , . C
Size: 1470px × 1699px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1840, booksubjectscience, bookyear1840
