ASTRONOMICAL APPARATUS—HALE. 969 
first slide on the screen, to say that I have some right to undertake 
a discussion of this sort, because I have viewed the subject from 
the standpoint of the man using small and inexpensive apparatus. 
In my first spectroscopic work, which was done in a room in my 
father’s house, the instruments were of the simplest character, and 
largely of my own construction. Later, a small building was con- 
structed for a concave grating of 10 feet focal length, and the ap- 
paratus, although powerful, was not expensive. Subsequently a 
tower and dome were added, and a 12-inch telescope was erected for 
photographie work upon the sun. After the preliminary experi- 
ments had been completed, and the spectroheliograph had begun to 
take form, the possibility that its results could be greatly improved 
through the use of a larger telescope suggested itself, and for this 
reason I made many efforts to acquire a large instrument for these 
solar investigations. The result, through the generosity of Mr. 
Yerkes, was the 40-inch. Yerkes telescope, which proved to be very 
useful for the extension of the spectroheliograph work. The next 
slide shows the instrument, which you will see is a large and expen- 
sive machine. The question, then, comes right down to this point: 
What are the advantages of such a telescope compared with, let 
us say, a 6-inch equatorial or possibly a 4-inch equatorial? Is it 
possible with a 6-inch equatorial to do work comparable in impor- 
tance with the work that can be done with a 40-inch equatorial ? 
The next slide will show that there was an advantage in passing 
from the Kenwood 12-inch to the Yerkes 40-inch, at least for the 
photography of the sun. Very minute details of the flocculi were 
brought out which had not previously been known. But it may 
easily be shown that the advantages of the 40-inch telescope for 
most classes of solar work are due more particularly to its great 
focal length than to its large aperture.* 
Let us take another illustration. Here we have a picture of the 
moon made by Professor Ritchey with the 12-inch Kenwood telescope. 
You will notice that near the terminator is the crater Theophilus, 
which you will see again in the next slide as photographed with the 
40-inch telescope. This photograph taken by Professor Ritchey is 
probably as good a photograph of the moon’s surface as has yet been 
made, and in this case the advantage of the 40-inch telescope is ap- 
parent. But if we take another case, as illustrated in the next slide, 
it becomes obvious enough that for certain classes of work the Yerkes 
telescope is not well suited. Here is a picture made with the 40-inch 
“So far as resolving power is concerned, an aperture of 8 inches would be 
sufficient to permit the smallest known details of the flocculi to be photographed. 
> Here, again, the full visual resolving power is not utilized, but the great 
aperture is of advantage in permitting the large image to be photographed with 
very short exposures. ; 
