1899] MICROSCOPICAL JOURNAL. 299 



tician of course would wish to do is to be able to make all 

 these colors bring their respective images to one and the 

 same focus on the axis of the lens, irrationality of the 

 spectrum being for the moment disregarded. In the man- 

 ufacture of achromatics however, this is an absolute im- 

 possibility, and the optician has to be content, so to speak, 

 with combining certain colors for visual purposes to give 

 the best resulting image — yellow or yellow-green for see- 

 ing with, and yellow and blue-violet for photographing 

 with, leaving all the others outstanding. Prof. Abbe 

 some years ago gave a large amount of attention to the 

 theoretical aspect of the case and came to the same con- 

 clusion as others had done before him, that no improve- 

 ment could be effected in achromatism, especially in the 

 elimination of the secondary spectrum, until the optician 

 was provided with new kinds of glass possessing proper- 

 ties at that time only dreamt of in theory. At length, 

 however, Jena glass was introduced, which gave to the 

 world many of the kinds of glass that had been wanted, 

 thus creating quite a revolution in lens- making. The words 

 "many kinds" are used, because for the present they do 

 not seem able to make a description having the same pe- 

 culiar properties possessed by the mineral "fluorite" up 

 to the present so absolutely necessary in the construction 

 of the apochromatic. Having the new kinds of glass at 

 command, Prof. Abbe originated his celebrated system of 

 Apochromatism, which, he says, differs essentially from 

 all others hitherto designed. Apochromats realize sim- 

 ultaneously two conditions not hitherto fulfilled by any 

 other optica! combination : (1) By the union of three 

 colors in one point of the axis which enables the optician 

 to eliminate the secondary spectrum ; and (2) the cor- 

 recting of spherical aberration for two different colors in 

 contradistinction to the hitherto usual correction for one 

 color only in the brightest part of the spectrum. 



To fix our ideas then, let us put the matter another way. 



