J, C. Draper—Dark Lines in the Solar Spectrum. 257 
all error that might arise during refraction was thus avoided. 
The mirrors of the heliostat were of flat glass silvered, the 
silver surface being polished served as the reflector. The sur- 
face of the concave mirror employed to bring the image of the 
slit to a focus, was also silvered and polished. Gratings of 
4800 and 9600 lines to the English inch, ruled on glass by a 
machine constructed by myself and my assistant Mr. Sickels, 
and also an admirable one of 17,280 lines to the inch, for 
which I am indebted to Mr. Rutherfurd, were used. These 
were silvered with a thin coating, and the unpolished silver 
surface employed to give spectra by reflection. it 
Power, was so adjusted as to make the leading lines of the 
photograph coincide with the same lines of Angstrom, drawn in 
their proper position below the scale as is shown in the diagram 
5 to O in the ultra-violet the leading lines of Cornu were 
employed. Among the advantages presented by this method 
of studying and measuring the lines of the spectrum we may 
In this way the effects of duration of seen and 
manner of development of the image, together with the varia- 
Hon in the size of the slit and focal distance may be investi- 
