AND A NEW METHOD OF ILLUMINATION. 365 



In the Navy, when Dreadnoughts were introduced, old-fashioned 

 battleships were scrapped ; so also in microscopical affairs those 

 who are wise will scrap all their dry lenses of powers higher than 

 3 in. or ^ in. 



One can foresee that the advent of this new lens means much, 

 for just as oil-immersions have eclipsed water-immersions, so will 

 this new lens supersede the wide-angled dry lens, which cannot 

 compete with it in working distance, quality, field, or price. 



It is to be hoped that Zeiss will issue an objective of this class 

 for the long as well as for the short tube. The most notable 

 feature in this new object glass is the near approach that has 

 been made towards apochromatism without the use of fluorspar. 



There is, however, another matter for your notice viz. an 

 entirely new way of using an object glass for diatom or other 



a 



Fig. 1. 



resolutions, a method, moreover, for which this new object glass 

 is peculiarly suited. The method is so simple that it can be 

 explained in a few words : (1) Place the diatom so that the striae 

 to be resolved are vertical in the field. (2) Set up a critical 

 imawe with the edge of the flame in focus and central to the 

 field, and open the diaphragm to its full extent. (3) By means 

 of the substage centring screws move the condenser so that the 

 image of the flame lies just outside the field of a high-power 

 eye- piece (fig. 1). If the striae are within the grip of the object 

 glass they will be resolved. 



It just amounts to this, that if one is working at diatoms 

 with critical illumination and has need to resolve one, all that is 

 necessary is to move the side way adjusting screw of the substage 

 and place the flame image just outside the field, and the thing is 

 done in an instant, without any trouble with stops, slots, or 

 other apparatus. 



You will notice that the amount of the displacement of the 

 condenser is very small (say twice the length of a Kavicida rhom- 



