410 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



the law, and there can be but little difficulty remaining in getting 

 the best results from a condenser. 



Fig. 348 illustrates another method of getting the same result. 

 We may illuminate a condenser with light direct from the name, as 



in fig. 347, or we may interpose the 

 f mirror as in fig. 348. M is the 



plane mirror, and, properly used, 

 exactly the same result may be 

 obtained as in the former case. It 

 is, however, slightly more difficult 

 to set up, but the method shown 

 in fig. 347 will, 'on the whole, be 

 preferable. 



Nothing can be of more moment 

 to the beginner than to understand 

 the practical use of the condenser. 

 We must direct the student to what 

 has been stated concerning it in 

 Chapter IV. But the following 

 should be carefully considered. 

 Fig. 349 shows a sub-stage con- 

 denser, S, and an objective, O, both focussed on the same, point. 

 The condenser has an aperture equal to that of the objective. Now 

 if the eye-piece be removed, and we look at the back lens of the 

 objective, it will be seen to be full of light, as at R. The same 

 thing, but with the aperture of the condenser cut down by a stop, is 

 seen in fig. 350. Now only a part of the back of the objective is 

 filled with light, as at T in the same illustration. 



Now it does not follow, because the back lens of the objective is 

 full of light, as in fig. 349, that therefore the field ought to be full of 

 light. The field only shows the bright image of the edge of the flame, 



M 



FIG. 348. Another method of getting 

 critical image. 



FIG. 349. Condenser and object-glass 

 with the same aperture. 



O 



FIG, 350. The same, with the aperture 

 of the condenser cut down. 



and it is in that alone that a critical picture can be found. If the 

 condenser be racked either within or without the focus, the whole 

 field will become illuminated, but at the same time a far smaller por- 

 tion of the objective will be utilised. On removing the eye-piece and 

 examining the back lens of the objective, pictures like D, H, fig. 345, 

 will be seen D when within, and H when without the focus. 



The condition represented in fig. 349 at R and O is the severest 

 test which can be applied to the microscopic objective ; that is to 

 say, to fill the whole objective with light and so test the marginal 

 and central portions at the same time. 



Even to obtain the state of illumination known as ' diffused day- 



