MICROSCOPE, ETC. 29 



In the rim E two screws are fitted, (only one is 

 represented in the figure for the sake of clearness) at 

 right angles to each other, opposite to thetn in the 

 interior of the tube a pretty strong spring S is 

 attached. 



By this means the ring A which fits on the pro- 

 jecting internal flanges of the rim R, can be moved 

 in any direction in the plane of the stage by the 

 screws, as the spring pressing against it on the 

 opposite side compels it to follow their motions. 



The diaphragm plate G is perforated with eight 

 holes varying from one half to one sixteenth of an 

 inch in diameter. In its centre is another aperture 

 quarter of an inch in diameter. Two thin plates 

 half an inch wide, through which the screw 8 fig. 1 6 

 passes, are placed above and below the diaphragm 

 before it is fastened in position, thus allowing it 

 to be displaced nearly quarter of an inch in its own 

 plane, and parallel to that of the stage, quite inde- 

 pendently of the remainder of the sub-stage apparatus. 



Any one possessed of ordinary skill and a few 

 tools for cutting sheet brass, will find little difficulty 

 in applying this principle to existing instruments. 



From what has been previously mentioned with 

 regard to the resolution of certain structural pecu- 

 liarities of microscopic objects, it follows, that their 

 accurate delineation depends as much on the cor- 

 rections of the condenser through which the light 

 passes to the object, for achromatism, and on the size 

 of its angular aperture, as it does on the perfection 

 of the objective in these particulars. 



