MICRO-OBJECTIVES AND COMPENSATING OCULARS. 155 



Schott, Dr. Abbe began in 1881 to make experi- 

 ments with the view of producing new kinds of 

 optical glasses. 



The relation of the optical properties to the 

 chemical compositions of various kinds of experi- 

 mentally produced glasses was first established by 

 spectroscopical researches, and finally such glasses 

 were produced as possessed the properties most 

 desired. In this way, and by combining a far larger 

 number of elements than formerly, especially by 

 means of phosphoric and boric acids, besides silicic 

 acid, two new and much desired results have been 

 attained — viz. : 



(1) The production of crown and flint glass in 

 which the dispersion for the different regions of 

 the spectrum shows approximately the same ratio, 

 and which, therefore, in achromatic combinations 

 permits of the almost complete elimination of the 

 secondary spectrum. 



(2) The increase of the number of optical media 

 in such a way that with the same mean refractive 

 index the dispersion, or with the same dispersion 

 the refractive index, may be varied within con- 

 siderable limits, especially so that high values of 

 the refractive index can be obtained, not as hitherto 

 only in combination with flint glass of high dis- 

 persion, but also with lower dispersive indices as 

 in crown glass. 



These new kinds of optical glasses are produced 

 in the Glastechnisches Laboratorium in Jena, which 

 has been supported in the most liberal manner by 

 the Prussian Ministry of Public Instruction, and 

 they are now being supplied to the trade. 



Dr. Carl Zeiss, who constructs all his lenses on 

 strictly scientific principles, and according to the 

 formulas of Professor Abbe, is the first optician 

 who has produced micro-objectives of these new 

 glasses, and he has thus been able to correct two 

 important defects which up to now could not be 

 overcome with the means at the disposal of opticians 



