74 



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. In this con- 

 nection 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 

 consists mainly of the large variety of glass with 

 different ratios of refractive index and dispersive 

 power. 



As defects of the objective have thus far been 

 specially mentioned, it will at this point be well to 

 state that the single lens of the eyepiece has also 

 been found unsuited and w r hile an achromatic lens 

 is unnecessary, a supplementary lens below and 

 near the upper lens has been beneficial in so 

 affecting the image, by collecting the rays, that it 

 can be viewed at one glance and without spherical 

 and chromatic aberration which the single lens 

 would show. It is for this reason called the field 

 lens or collective and the upper lens the eye lens. 

 Both of these lenses are mounted so as to form one 

 part of the microscope with fixed relations and is 



