EUGENE SCHNEIDER 97 



M et alloy raphij. — In metallography the objective serves as con- 

 denser. The illumination thus obtained may be perfect (especially 

 with apochromatics), but the lenses are more or less marked, which 

 somewhat impairs th^eir definition. In any case, it is the illumina- 

 tion which will have to be studied for improvements. The problem 

 appears to be singularly arduous, and a long time will no doubt 

 elapse before the introduction of notable perfections can be hoped 

 for. 



Coiichision. — In a general way mechanical perfections of the 

 microscope will naturally result from progress in micrographic tech- 

 nics. From the optical point of view we are restricted, at least in 

 usual practice, by the impossibility of going beyond the numerical 

 aperture of 1.40. Better correction of the aberrations and especially 

 of the field curvature seem only to be possible by the creation of new 

 optical materials. Finally, the use of ultra-violet rays admits of in- 

 creasing the definition to a considerable degree; but the insufficient 

 transparency of the media frequently imposes a limit. 



