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 



