Vol. 6, 1920 
ASTRONOMY: H. SHAPLEY 
127 
constant n. In the formula as given in the paper referred to the coefficients C and the 
exponents h are functions of the temperature. The stress P is constant and p is approxi- 
mately one-half. 
^ It was found by experiment that for stresses not too great, the "direct" curve 
(on applying the stress) and the "return" curve (on releasing) were the same; or rather 
if the former is 5 and the latter R, then 5 — R = Ct. 
NOTE ON THE SIMPLE DEVICE FOR INCREASING A PHOTO- 
GRAPHIC POWER OF LARGE TELESCOPES 
By Harlow Shapl^y 
Mount Wii^son Observatory, Carnbgie Institution of Washington 
Communicated by G. E. Hale, February 5, 1920 
Among the many problems in sidereal astronomy that demand great 
telescopic power the three following may be cited as particularly significant 
for inquiries relating to the origin and evolution of stellar and galactic sys- 
tems: (1) The total number of stars and limiting faintest magnitude in 
globular clusters; (2) the frequency and distribution of dwarf stars in 
clusters and in the sky at large ; (3) the limits of the galactic system in cer- 
tain directions. To these stellar problems we should add the highly in- 
teresting questions connected with the faint extra-galactic nebulae, and 
note that important contributions toward their solution seem to be only 
a little beyond our present telescopic power. Thus the best available 
photographs of globular clusters suggest that we are approaching the 
faintest magnitudes, and that, if we could only extend our studies two or 
three magnitudes farther, one phase of the problem of dwarf stars could 
probably be solved. 
The longest exposures with the 60-inch reflector have yielded apparent 
photographic magnitudes between 20 and 21. The Hooker telescope, it 
is believed, will gain about a magnitude over the 60-inch provided the 
focal images are not much larger; but in the case of nebulae, since the 
ratio of focal length to aperture is the same for the two reflectors, 
no gain is to be expected except in scale, which is of course important for 
revealing new structure and purposes of measurement, but will not bring 
fainter structure to view. Small nuclei will also be shown in stronger con- 
trast with the 100 inch telescope. 
The investigation of faint stars and nebulae is of sufficient importance 
to justify every attempt to increase beyond normal limits the working 
range of great reflectors. Since these instruments are universally adapted 
to photographic observation, the problem is one of increasing the brightness 
of the image or increasing the sensitivity of the photographic plate. 
In the device described below the principle employed is to increase the 
brightness of the image by reducing its area. This is accomplished by 
placing a short focus lens in the converging beam at an appropriate dis- 
tance in front of the photographic plate. The immediate result is to re- 
