ASTRONOMICAL APPARATUS—HALE. O71 
and in doing so ( shall confine myself almost entirely to observa- 
tions of the sun. although one might attack the subject from many 
other directions. The first point is this: Suppose one has a small 
telescope of 4 ches or 6 inches aperture and wishes to observe 
the sun with it; and let us assume at the outset that he has no attach- 
ments whatever in the form of spectroscopes, but that he wishes 
simply to make direct observations of the sun. Js there work for 
such an instrument at the present time? If you will examine the 
literature of the subject you may perhaps be surprised to find that 
many years have elapsed since very careful and extensive investiga- 
tions have 2en made similar to those of Langley, which may be 
almost forgotten by many astronomers, but certainly are not for- 
gotten by those of us who follow the sun and are accustomed to the 
appearance of the spots when the definition is good. The next slide 
shows the well-known drawing of Langley’s typical sun spot. (See 
plate m1.) You will remember, if you have systematically observed the 
sup, that every time the conditions become extremely good, the struc- 
t- eof sun spots more and more closely resembles this drawing. This 
is a typical drawing; it does not represent any particular spot; it 
bvings together observations of various spots; but in general the details 
of as i spot look very much indeed like that drawing when the defi- 
nition is good enough to show them properly. This subject has been 
s~satly neglected for a long time, and it would well repay observers 
with large or small instruments to observe sun spots, and to study 
many of the details of their structure which still remain obscure and 
difficult to understand. Of course the question of the resolving 
power of the instrument must then be considered. A 4-inch telescope, 
capable of separating objects one second of arc apart, would not do 
for the very finest details in a sun spot. According to Langley, the 
penumbral filaments sometimes exhibit structure considerably smaller 
than such a telescope would show; but a 10-inch or 12-inch telescope 
would show everything that has ever beer recorded in a sun spot, 
nnd there are many instruments of that size available for such 
observations.” Even a much smaller telescope, if carefully and sys- 
tematically used, would contribute largely to our knowledge of stn 
@Por example, it would be of great interest to study the structure of the 
umbra, as seen through a minute pin hole in the focal plane of a positive eye- 
piece, aS Dawes did many years ago. 
bTt must not be forgotten that photography is still behind visual observations 
in revealing the minute structure of sun spots. It can hardly be doubted, how- 
ever, that if only the umbra and penumbra were permitted to fall on the plate, 
and the exposure properly regulated, new and valuable results weuld be ob- 
tained. The amateur will readily find many opportunities for work in this 
field. 
