824 Transactions of the Society. 



microscopic objectives a useful improvement of the dioptric per- 

 formance (definition) is obtained even when the defect of secondary 

 chromatism cannot be removed. 



3. The homogeneous immersion system admits of a useful 

 increase of aperture closely approaching the ultimate limit which 

 is imposed by the optical qualities of the materials hitherto available 

 for microscopic preparations. 



