Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

336 OPTICAL PROJECTION spectra by reflection; but much more brilliant spectra are projected if it be silvered, or by gratings ruled on speculum metal. Formerly the best of these were by Mr. Rutherford, of New York, but for years past those ruled by a machine constructed by Prof. H. Rowland have surpassed all others. The gauge of these ranges from 10,000 to 30,000 lines to the inch, the 17,000 gauge being about the best; and they can be obtained from Mr. W. Brashear, of Philadelphia, of various sizes; and either ruled on a plane surface, or one slightly concave, so as to give a focus without lenses. The condition of projection is exactly the same as with transparent gratings; the slit must be focussed on the screen as reflected by the surface, and only diffracted by the ruling. One of those metal buttons con- structed by the late Sir John Barton, and known still as ' Barton's buttons,' whose surface is divided into hexa- gonal or other portions, each of which is ruled with fine lines in a different direction from its neighbours, will yield beautiful projections, the spectra being all arranged in the directions of a six-pointed star. The ' focussed parallel beam' from a small aperture is intercepted by the grating, which reflects the spectra on the screen. A fine piece of polished mother-of-pearl also gives beauti- ful diffraction colours by reflection, the colours shifting as the plane of the pearl is a little altered in position. The easiest way of doing this is to provide a small tablet of thin blackened wood, D, with a boss, B (fig. 184), on the back, into which is fixed a tube, T, fitting into the sockets of the pillar-stands. The pearl, or any other object, such as a small haliotis shell, is easily held on this by a couple of elastic bands, A peacock- Fio. 184