. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. 15 ference is therefore impossible. Again, if the spectrum lines are in coincidence, the longitudinal axes usually diverge by a small angle. Furthermore, the interferences are almost always eccentric and the lines hair-like, indicating distant centers. I have not succeeded in making a perfect adjustment, systematically; but the discrepancies indicated are themselves interesting in their bearing on the subject of this paper. In figures 7 and 8, L is a vertical blade of white light from a collimator with fine sl
. Carnegie Institution of Washington publication. REVERSED AND NON-REVERSED SPECTRA. 15 ference is therefore impossible. Again, if the spectrum lines are in coincidence, the longitudinal axes usually diverge by a small angle. Furthermore, the interferences are almost always eccentric and the lines hair-like, indicating distant centers. I have not succeeded in making a perfect adjustment, systematically; but the discrepancies indicated are themselves interesting in their bearing on the subject of this paper. In figures 7 and 8, L is a vertical blade of white light from a collimator with fine slit, and G is the grating. The two first-order spectra leaving the ruled face at the line g strike the opaque mirrors M and N, the former on a micrometer moving the mirror parallel to itself. From M and N the rays reach the half-silvered plate of glass HS, where one is transmitted and the other reflected into the telescope T. The coincident rays R are superfluous. After placing the parts and roughly adjusting them for symmetry with sunlight, the finer adjustment may be undertaken. It may be noticed that the two systems M and N, and G as well as HS, can be used for further adjustment separately. All are provided with adjustment screws relatively to rectangular axes. To put the mirrors M and N in parallel and in the vertical plane with the grating G, the half-silvered plate should be removed /. 10 and replaced by a small white vertical screen of cardboard, placed at right angles to the direction of HS in figure 7 and receiving both spectra. A fine wire is drawn across the slit to locate the longitudinal axis, and an extra lens may be added to the collimator and properly spaced until the doublet insures sharp focussing. Both mirrors, M and N, are now rotated on hori- zontal axes, until the longitudinal black lines in their spectra cease to diverge and coincide accurately. G, M, N, may now be considered in adjustment. On returning the half-silvered plate, HS, it in turn is to be car
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