200-INCH HALE TELESCOPE—HUBBLE 179 
eround of large, well-known markings, have been recorded by various 
trained visual observers, using telescopes of all sizes from 6 inches up- 
ward, during the whole of the past 70 years since Schiaparelli first 
reported them. ‘The canal systems, if real, would almost necessarily 
imply the existence now or in the past of intelligent beings on Mars. 
But other trained observers, again using telescopes of all sizes, 
report no trace of canals. KE. E. Barnard, perhaps the greatest of the 
American visual observers, studied the planet over many years with 
the then largest telescopes in the world, including the 36-inch refractor 
at Lick, the 40-inch at Yerkes, and the 60-inch reflector at Mount 
Wilson, and, although he saw an immense amount of detail, he found 
no canals. 
The two groups of observers flatly contradict each other, and since 
the observations are personal impressions neither group can demon- 
strate the validity of its assertions. Evidently the controversy must 
be resolved by photography. Once photographs are available on which 
the canals should appear if they are real features of the planet, the 
question will be settled beyond reasonable doubt. The test has not 
been possible as yet because existing equipment, although closely ap- 
proaching the required standards, does not fulfill them. The 200- 
inch, however, should meet all the necessary conditions and settle the 
question. 
The problem is as follows: Mars is a small object. The image at 
the primary focus of the 100-inch is less than 5 inch at the most 
favorable oppositions, and less than % inch at the long Cassegrain 
focus. In order to get an image large enough to serve as a critical 
test of fine markings (i. e., to make the resolution of the photographic 
plate comparable with the optical resolution of the telescope) it is 
necessary to use an enlarging lens. Thus the total light collected by 
the telescope is spread over a much larger, and correspondingly fainter, 
image. Furthermore, because of the atmosphere on Mars, it is neces- 
sary to photograph the surface markings through deep orange or red 
filters, still further reducing the effective brightness of the image. 
The reduction is so great that photographs with existing telescopes 
require time exposures instead of snapshots. The exposures may be 
only a second (or even a fraction of a second, with the 100-inch) but 
they are long enough to permit the dancing or shimmering of the 
image to smear out the finest detail. 
You doubtless realize that a telescope magnifies the twinkling of 
stars along with everything else. Critical observations are restricted 
to periods of maximum steadiness (good seeing, as it is called), and 
even then, the shimmer is appreciable under high magnification. The 
eye can “hold”? an image under these conditions, but photography 
with time exposures is helpless. The shimmer smears out the fine 
details. It is for this reason that, in the case of fine markings such 
