Masterpieces of Science 



enough. During four hours ending at two 

 o'clock in the morning, M. Zenger has taken 

 photographs of Lake Geneva and Mont Blanc 

 when nothing was perceptible to the eye. Turned 

 to the heavens, this power to grasp the invisible 

 brings into view a breadth of the universe unseen 

 by the acutest observer using the most powerful 

 telescope. Let the lenses of such an instrument 

 be directed to a definite point in the sky by 

 accurate machinery, and they will maintain 

 their gaze with accumulating effect upon a 

 sensitive plate through all the hours of a long 

 night, and, if need be, will renew their task the 

 next night, and the next. 



In this work the utmost mechanical precision 

 is imperative. Professor E. E. Barnard says 

 that if the motion of a guiding clock varies as 

 much as one-thousandth of an inch during an 

 exposure of from three to eight hours, the images 

 are spoiled and worthless. It was only after 

 repeated failure that mechanicians were able to 

 make a clock sufficiently accurate to keep a star 

 image at one fixed point on a plate. Steadily 

 caught at one unchanging place, a ray, however 

 feeble, goes on impressing the pellicle of a plate, 

 minute after minute, hour after hour, night after 

 night, until at last, by sheer persistence, the 

 light from a star or a nebula too faint to be de- 

 tected in a telescope imprints its image. Some 

 images have been obtained as the result of twenty- 

 five hours' exposure during ten successive nights, 

 so as to get impressions from as near the zenith 

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