276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
are of wood, built in a few hours by a carpenter. The wooden sup- 
port for slit and plate holder stand on a concrete pier, and close an 
opening through a partition which forms one end of a narrow dark 
room. Eighteen feet from the slit, within the dark room, is another 
concrete pier. A sliding wooden support, carrying a lens, and a 
simple wooden mounting for the grating, stand on this pier, and com- 
plete the spectrograph. (A similar spectrograph, suitable for use in 
an open room, is illustrated in plate v.) Owing to the scarcity of 
gratings, we are fortunate in being able to use one loaned by Professor 
Ames, of Johns Hopkins University. If we had no reflecting grating, 
we could buy a replica very cheaply from Thorp, or Wallace, or 
Ives,’ which would give quite as good photographs as we obtain now 
(though the exposures would be longer, because of the smaller aper- 
ture). They might even be better, because our photographs of 
spot spectra (made with the similar spectrograph of the Snow tele- 
scope) are not what they ought to be, or what I hope they will sub- 
sequently become.° They would not stand comparison for a moment, 
so far as perfection of definition is concerned, with those magnificent 
photographs of the solar spectrum made by Mr. Higgs in the center 
of Liverpool, under conditions which would ordinarily be called 
very bad even for a crowded city, with tram cars constantly passing 
in front of the house. With a spectrograph of his own construction 
(except the grating), Higgs made the finest photographs of the 
solar spectrum ever produced ; superior, as Rowland would have said, 
to the best photographs made by himself at the Johns Hopkins 
University. It is obvious that something other than an expensive 
instrument is required to make a good photograph. Mr. Higgs has 
the ability, which others may acquire, to obtain superb definition 
and exquisite photographs with very simple apparatus indeed. 
With a spectrograph of 1 inch aperture and 10 feet focal length, 
used with a fixed telescope of 4 inches aperture and 40 feet focal 
length, one would be in a position to make good photographs of the 
spectra of sun spots. 
What, then, are the relative advantages of visual and of photo- 
graphic work? The next slide shows some photographs. The upper 
one is the spectrum of the sun and the lower one is that of a spot. 
These photographs are better than visual observations for the deter- 
mination of the wave lengths of unknown lines in spot spectra, simply 
“Except the plate holder, which is of a standard make. 
®*As these replicas are not reflecting gratings, the auto-collimating spectro- 
graph might in this case give way to one in which a separate camera lens is 
used. With the angular aperture here considered, well-made simple lenses would 
obviously serve perfectly well for collimator and camera, the photographic plate 
being set at the angle required to bring a sufficient range of spectrum into focus. 
¢ See footnote on p, 273. 
