objectives, we are much more indebted for his profound 

 disclosure of the principles of microscopical optics, 

 as well as to the combined efforts of himself and 

 Dr. Schott for their labors in the art of glass making 

 and to the large variety of glass which they have 

 placed at the disposal of opticians, who by this means 

 have been able to accomplish much higher results 

 than would otherwise have been the case. All of the 

 ordinary kinds of glass, faultless as they may appear, 

 are entirely unsuitable for use in the production of the 

 higher grade of optical instruments. The production 

 of optical glass for this use is fraught with many diffi- 

 culties. It must be absolutely homogeneous, free from 

 impurities and unchangeable under varying climatic 

 conditions. The mere fact of the existence of air 

 bubbles, while undesirable, is not necessarily a defect, 

 as it has been found impossible to produce certain 

 kinds of glass without them. 



The amount of injury due to air bubbles is in the 

 loss of light, and as microscope lenses are small the 

 percentage of this loss may be considerable, therefore 

 bubbles are carefully avoided. 



In this connection it is opportune to state that the 

 production of this glass, generally termed Jena glass, 

 has been taken advantage of by unscrupulous parties 

 in creating the impression that the bare fact of using 

 this glass gives in itself much better results. Such is 

 not at all the case. The merit of the production con- 



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