202 PROGRESS OF SCIENCE IN THE CENTURY. 



tine dry plate, which on sufficiently long exposure 

 will register an image of a body whose luminosity 

 falls far below the limit of visibility to our eyes, 

 has meant a remarkable extension of our sense of 

 sight It has meant seeing the invisible ! 



Of some importance, too, has been the develop- 

 ment of more exact methods of measuring star bright- 

 ness (photometry), and the resulting classification 

 (first suggested by Pogson in 1856) into definite de- 

 grees of "magnitude." Thus a star of the sixth 

 magnitude is one hundred times fainter than one of 

 the first magnitude. 



Intensifying of Observation. Inspection of the 

 recent moon-maps and photographs, as seen, for in- 

 stance, at the Paris Exposition (1900), will illus- 

 trate what is meant by an intensifying of observa- 

 tion. 



The Moon. The large size of our satellite (2,160 

 miles in diameter), and its relative nearness to us 

 (238,833 miles from the earth's centre), facilitated 

 the careful study of its superficial characters, at 

 least of that side which is alone presented to our 

 view. The systematic and interpretative study of 

 the moon's face practically began with the century, 

 for it dates from Schrotcr's Solenotopographische 

 Fragmente (1791-1802). Lohrmann and Schmidt, 

 Beer and Miidler, Nasmyth and Carpenter, Neison 

 and Secchi, and many more have added detail to de- 

 tail, so that it is safe to say there is no country 

 mapped so nearly up to the present limits of possible 

 precision. The heights of some of its mountain 

 ranges have been computed from their shadows and 

 the depths of some of iU extinct craters have been 

 sounded. We have certainly advanced far from the 

 older view, which even Herschel did not entirely get 



