274 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
sideration is a small one. But if the spot is a fairly large one (and 
hitherto no one has had time to observe the spectra of small spots sys- 
tematically), I think there is no advantage whatever in having a large 
telescope to form the image of the sun on the slit of the spectroscope ; 
it is merely a question of having an image of moderate dimensions 
upon the slit, and after that the spectroscope does the work. So that, 
so far as the larger spots are concerned, a small telescope is quite as 
satisfactory as a large one for viswal work on their spectra. 
T will return in a moment to the question of the relative advantages 
of the photographic and the visual methods of observing spot spectra 3 
but I want to point out in passing that the 40-inch telescope has cer- 
tain very definite advantages for work on the sun. If one wishes to 
observe the spectrum of the chromosphere, for example, the advan- 
tages of great focal length immediately become apparent. The width 
of the spectroscope slit is essentially constant; the chromospheric arc 
must have a certain linear width on the slit in order to permit the 
base of the chromosphere to be observed; and consequently the spec- 
trum of the chromosphere, as seen with the 40-inch telescope, is a re- 
markable sight, showing thousands of lines which do not come out 
with a small focal image of the sun. 
Here we have, then, an illustration of the advantages for certain 
purposes of considerable focal length. I think it is not so much a 
question of the telescope’s aperture here, because we must not forget, 
in thinking of the optics of this question, that the brightness of the 
spectrum (for constant purity) is quite independent of the linear or 
the angular aperture of the object-glass that forms the image of the 
sun on the slit of the spectroscope.?. Perhaps it is well to bear in 
mind that the brightest solar spectrum one can get is obtained with- 
out any telescope whatever to form an image on the slit, but merely 
with a collimator of suitable angular aperture. But a large solar 
image is frequently advantageous, and an equatorial telescope of great 
focal length is necessarily an expensive instrument. The aperture 
in the case just mentioned is less important than the focal length; but 
even if the aperture were only 6 inches and the focal length un- 
changed, the tube must still be 64 feet long, and the mounting would 
cost no less than the mounting of the Yerkes telescope. So if we 
wish to have an instrument of great focal length, and yet keep down 
@ When the focal length of the collimator is limited (as is usually the case in 
a spectroscope attached to an equatorial telescope), an increase in the angular 
aperture of the telescope permits the linear aperture of the spectroscope, and 
consequently the resolving power and the brightness of the spectrum, to be 
increased up to a limit fixed by the size of the grating available. With a celo- 
stat telescope, however, the same conditions do not obtain, since the aperture 
of the spectroscope can be increased by merely increasing the focal length of the 
collimator, 
