Masterpieces of Science 



hardly more, indeed, than a mere "first approx- 

 imation" to absolute perfection. The fault is 

 not with the optician, but in the material with 

 which he has to deal. 



The kinds of glass hitherto at his disposal are 

 such that it is impossible to make from them 

 lenses which will bring to the same focal point 

 more than two differently coloured rays of the 

 spectrum. If, for instance, the red and blue are 

 perfectly united, then the green rays will come 

 to their focus nearer to the lens and the violet 

 farther away. In the best of the great telescopes 

 now existing, therefore, the image of a bright 

 object is surrounded by a strong purplish halo, 

 which to the uninitiated appears very beautiful, 

 but which to the astronomer is an abomination, 

 because it makes it difficult to see small objects 

 near the bright one, and seriously injures the 

 definition of details upon the disk of a planet. 



Within the past few years, however, the Ger- 

 man manufacturers at Jena, working with a 

 government subsidy, have been able to produce 

 new kinds of glass which, properly combined, 

 gives lenses free from this fundamental defect and 

 have enabled their opticians to obtain unprece- 

 dented perfection in the construction of micro- 

 scopes. Hitherto it has not been found practi- 

 cable to supply disks of large size sufficiently 

 homogeneous for telescope lenses and at the same 

 time of a quality to resist the atmospheric hard- 

 ships to which such lenses are necessarily ex- 

 posed. But progress is constantly making. A 

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