ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 



715 



only of tlie piano mirror m. Fig. 157 gives results as critical as fig. 156, 

 it is, however, a little more troublesome to set up, and therefore 

 fig. 156 will be found preferable where the instrument is sufficiently 

 tucked up on its trunnions to permit of its being so used. 



Fig. 158 A shows a 

 substage condenser S, and 

 an objective 0, focused on 

 the same point ; the con- 

 denser being of an aperture 

 equal to that of the objec- 

 tive. On removing the eye- 

 piece and looking at the 

 back lens of the objective, 

 it will be seen to be full of 

 light as at C. 



Fig. 158 B shows the 

 same thing, but with the 

 aperture of the condenser 

 cut down by a stop. Now 

 only a portion of the back 

 lens of the objective is filled 

 with light. (See D.) 



It does not follow that 

 because the back lens of 

 the objective is full of light, 

 as at fig. 158 C, that there- 

 fore the field ought to be 

 full of light. The field only 

 shows a bright image of the 

 edge of the flame ; but it 

 is in the plane of that 

 image where the picture is 

 critical. 



If the condenser be 

 racked either within or 

 without the focus, the whole 

 field will become illumi- 

 nated. At the same time, 

 however, a far smaller por- 

 tion of the objective will be 

 utilized. On removing the 



eye-piece, and examining the back lens of the objective, a picture like 

 fig. 154 C, p. 714, will be seen. 



Fig. 158 A shows the most severe test that can be applied to a 

 Microscope objective, viz. to fill the whole of the objective with light, 

 and 80 test the marginal and central portions at the same time. Few, 

 indeed, are the objectives that will stand this ordeal. Some fog 

 when half full of light ; most when one-third full ; and not one in one 

 hundred will bear three-quarter filling. 



We now come to some very obvious points — so obvious, indeed. 



Fig. 158. 



o ^ 



B 



