126 HOW TO SEE WITH THE MICROSCOPE. 



a corresponding- increase of definition, when worked by 

 xtrem<>ly oblique light over balsam mounts of " diffi- 

 cult tests." These characteristic differences in perform- 

 ance are so palpable, as to enable us to select in less 

 than fifteen minutes' use of two glasses over the Moller 

 probe plate, the higher-angled glass (balsam) from the 

 lower. 



It has been claimed by some who have used the duplex 

 glasses, that the higher performance by central light is 

 obtained with those of tho smaller balsam angle. My 

 own experience does not authorize me to endorse these 

 conclusions. Certain it is, that when the higher angles 

 are used by central illumination, their immense power 

 of light will, if the matter receive not proper atten- 

 tion, defeat the glass, and again, even with oblique 

 illumination, the high-angled objectives require the 

 most careful attention and expert handling. 



We are, therefore, prepared to endorse to some ex- 

 tent, referring to high balsam angles, the remarks which 

 have been quoted from " The Microscope and Its Reve- 

 lations." 



FLATNESS OF FIELD. 



It is, of course, desirable that an object known to be 

 flat should so appear when viewed under an objective. 

 The optician, however, has thus far found it impossible 

 to secure perfection in this respect, combined with the 

 highest aperture obtainable, and this might be urged as 

 an objection to the use of wide-angled objectives. The 

 slight err or shown by the glasses referred to, has much 



