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THE POPULAR SCIENCE MONTHLY 



Sketches of the Planet Venus. 

 The sketches at the top were made on November 1, 1907, by Dr. Lowell (on the 

 left), and on November 2, 1907, by V. M. Slipher (on the right). The sketches in the 

 middle were made by V. M. Slipher and E. C. Slipher, on February 27, 1909. The 

 sketches at the bottom by Dr. Lowell and K. C. Slipher, on April 12, 1909. 



Light is due to wave-motion of the ether, and ordinary light consists 

 of a mixture of light of various wave-lengths. By means of a prism or 

 grating these are refracted differently and so sifted into a colored rib- 

 bon or band, the longer waves lying at the red end of the spectrum, as 

 the ribbon is called, the shorter at the violet. Now the spectroscope is 

 such a prism or grating placed between the image and the observer, by 

 means of which a series of colored images of the object are produced. 

 In order that these may not overlap and so confuse one another, the 

 light is allowed to enter the prism only through a narrow slit placed 

 across the telescopic image of the object to be examined. Thus sue- 



