ASTRONOMICAL APPARATUS—HALE. 275 
the expense to a reasonable figure, we must use a telescope of a differ- 
ent type. There are many other reasons why we should wish to use a 
fixed telescope for certain kinds of solar work, although I should be 
the last to admit that the 40-inch telescope is not an almost perfectly 
satisfactory machine of its kind. It has, as we have seen, inconveni- 
ences and disadvantages for some classes of work, but in other fields 
its superior qualities become more and more striking day after day 
as the observer learns to appreciate them. I only wish we could afford 
to have such a telescope (or even a much smaller equatorial refractor) 
on Mount Wilson, as it would be of great service for many purposes. 
Now let us consider some of the possibilities of the fixed telescope; 
and let me show, for purposes of comparison, a picture on the screen 
of the Snow telescope which is now employed at Mount Wilson 
(plate m1). Here is a celostat, with mirror 30 inches in diameter. 
After passing to a second mirror the light is reflected to a concave 
mirror of 60 feet focal length (plate 1v), which sends it back and 
forms a large image of the sun within a laboratory. This is a very 
simple instrument indeed. The first ccelostat we set up on Mount 
Wilson was a small one used by the Yerkes Observatory party at the 
eclipse of 1900, and it was not originally arranged for work of this 
kind; so we simply built a wooden support for a second mirror, and 
with the aid of a 6-inch objective of 60 feet focal length we made a 
telescope which served admirably for our solar work until this one 
was put up on the mountain. 
The next photograph shows the spectograph used with the Snow 
telescope. It is of the Littrow or autocollimating type, with slit 
and plate holder at one end of a long tube and lens and grating at 
the other. Light from the solar image, after passing through the 
slit, falls on the lens 18 feet (its focal length) distant. The rays, 
thus rendered parallel, then strike the grating and are returned to 
the lens, which forms an image of the spectrum on the photographic 
plate, just above the slit (the grating being tipped back a little). 
Such an outfit (fixed telescope and spectrograph) is an extremely sim- 
ple thing to build in inexpensive form. Ccelostats, for example, are 
common nowadays for eclipse work. One might have a ccelostat with 
a mirror only 6 inches in diameter and a second mirror about 4 inches 
in diameter, and then perhaps a telescope lens of 4 inches aperture 
and 40 feet focal length. Such an instrument as that, which could be 
built very cheaply indeed, would give a large solar image, adapted 
for many kinds of solar work. 
Let me show you in the next slide how we build our spectrographs 
in actual practice. This is the most powerful spectrograph in use 
in the laboratories of the Solar Observatory. Here is a little slit 
I bought from Hilger, the last time I was in London, for a few shil- 
lings. All other parts of the spectrograph, except a lens and grating, 
