518 



J. W. GORDON ON A " NEW " OBJECT GLASS BY ZEISS. 



use with any dry lens as required, of which it will increase the 

 magnifying power by 50 per cent, with a proportionate increase 

 in the amount of light collected, so that the enlarged image loses 

 nothing in brightness. In this sense it increases the resolving 

 power of the system, inasmuch as it increases the scale upon 

 which all the details are shown. 



That, however, is but the least part of its merit. If that were 

 all it would only constitute a |-in. objective the equivalent of 

 ^ in. ; I in. the equivalent of i in., and so on. What is very 

 much more important is that it gets rid of the top light reflected 

 down upon the surface of the object by the upper surface of the 

 cover glass. It does not seem to be at all generally understood 

 by microscopists how much resolving power is lost by reason of 

 the fog produced by these reflections from the upper surface of 



COVER GLASS. 



Fig. 2. 



the cover glass. When we are examining an object like a 

 diatom by transmitted light and producing, by means of that 

 transmitted light, what the late Prof. Abbe used to call an 

 absorption image that is to say, an image in which the face 

 presented to us by the object is seen in shadow it is of first-rate 

 importance that the shadows should not be illuminated by top 

 light. A dry cover glass sends back upon the upper surface of 

 the object a large amount of such top light, thereby obscuring the 

 contrast by virtue of which the image is seen. Furthermore, if 

 the object is not in actual optical contact with the glass, there 

 is sure to be a reflecting surface between the glass and the 

 mounting medium which is illuminated by this top light and 

 again produces a brilliant haze through which the object has to 

 be viewed. The diagram fig. 2 will serve to illustrate these 

 points. Here a section is taken through the cover glass with 



