156 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



and which offered insurmountable obstacles to 

 the further improvements in lenses. In con- 

 sequence, namely, of the great disproportion of the 

 dispersion of the various colours of the spectrum, 

 a property inherent in crown and flint glass, our 

 best so-called achromatic lenses have up to now 

 been corrected for only two colours of the spectrum, 

 and the hitherto unavoidable remnant of unachroma- 

 tism, the so-called secondary spectrum, was always 

 more or less perceptible. With the crown and flint 

 glass used by opticians it was equally impossible 

 to correct the spherical aberration for more than 

 one colour. All objectives, although fairly well 

 corrected for the middle of the spectrum, showed, 

 nevertheless, a spherical under-correction for the 

 red and a spherical over-correction for the blue 

 and violet rays, which imperfection appeared as a 

 more or less great inequality of the achromatic 

 correction between the central portion and the 

 peripheral zones of the objectives. 



These defects caused an imperfect combination 

 of the image-forming rays, and as a result objec- 

 tives, especially those of large apertures, did not 

 allow the employment of high magnifying oculars, 

 because those deficiencies of spherical and achro- 

 matic corrections became more apparent with them 

 than with the lower magnifying ones. 



The practical advantages of Abbe's new objec- 

 tives made by Carl Zeiss from the new optical 

 glasses are the following : — 



(1) The full value of the large apertures of objec- 

 tives becomes now only apparent, because owing to 

 their perfect corrections, the images formed by the 

 new dry and water immersion objectives are scarcely 

 distinguishable from those formed respectively by 

 the water and homogeneous immersion objectives 

 of perceptibly larger numerical aperture hitherto 

 constructed. 



(2) The largest magnifications for a certain aper- 

 ture can be obtained by high eyepiecing and by 



