V. Note on a new form of direct vision spectroscope

Direct vision spectroscopes are very useful in the observation of shifting objects, such as auroræ and other meteors. They are generally in request for telescopic work, and also in all cases where rapidity of observation is of consequence. Ordinary direct vision spectroscopes with compound prisms have the disadvantage that the dispersion of the red end of the spectrum is small; less in proportion to that of the blue end than in spectroscopes with simple prisms. Also no measurements of lines can be made with them, except by means of a scale fixed in the field of view, which it is often difficult to see for want of illumination. Some time since (‘Roy. Soc. Proc.,' vol. 28, p. 482) we brought under the notice of the Society a direct vision spectroscope on Thollon’s plan, which had not the faults of the instruments with compound prisms. It gave a dispersion equal to that of two prisms of 60°, and excellent definition, but the number of reflecting and refracting surfaces which had to be truly wrought was rather large, and the movement of the prisms by a screw made measurements with it slow.