I30 
ASTRONOMY: H. SHAPLEY 
Proc. N. a. S. 
inches and the largest aperture possible, and should not be wasteful of 
light through reflections from numerous glass-air surfaces. If an extended 
object, rather than a point source, is to be photographed, the flatness 
of the intensifier field is important ; or, better still, its focal properties should 
be such as to improve the field of the primary. 
Putting S = F/A and s = f/a for the ratios of focal length to aperture 
for the objective (mirror) and intensifier, respectively, we find the mini- 
mum equivalent focal length, which is obtained when the cone of light 
from a source on the axis exactly fills the intensifier, is 
F, 
min 
1 + S/s' 
also 
For the Mount Wilson reflectors 5 = 5, and the greatest reduction of scale, 
for full accommodation of the convergent light, is 
1 
R 
min 
1 + 5A 
An intensifying lens with focal ratio 1.0 would, at the limit, increase the 
theoretical photographic limit by nearly four magnitudes for a source 
on the axis. 
Intensifying devices are particularly suited to reflecting telescopes 
because of the freedom from chromatic aberrations; also they should be 
of especial value when used with instruments of largest aperture and longest 
focal length because in studies of faint nebulae the large scale allows the 
minification necessar)^ for increased speed. Influences of bad seeing, or 
any other defects in the photographic images, are obviously minimized 
with the intensifier — a fact that may make it possible to use large reflectors 
under conditions otherwise impracticable. 
The lens now in use is a Dallmeyer No. 2 Kinematograph of 3 inches 
focal length and focal ratio 1.9. It has a fine field, but the many glass- 
air reflections considerably diminish the light. The lens is mounted so 
that the reduction can be readily changed, with a range of R between 
^/v and ^/t at the primary focus of the 100-inch reflector. For R = V2 
the workable field is about 6' in diameter — quite sufficient for most clusters 
and nebulae ; f or = Vs it is one third as large. A comparative discussion 
of results obtained will be made in a later communication. The prelim- 
inary observations, which include successive exposures on the same plate 
with and without the intensifier, show that, when a reasonable allowance 
is made for loss of light in the lens, the predicted results are fully obtained, 
at least for nebulae. For instance, a three minute photograph of the spiral 
Messier 77, with a reduction of 'Vs, shows much fainter nebulosity than 
a ten minute exposure without the intensifier; and in the star cluster 
Messier 3 a reduction of ^/ 15 gives a gain of nearly a magnitude for exposures 
of equal length. 
