Principles of Microscopy 11 



this cannot be increased unless the angle of the cone is increased. This 

 is the function of the substage condenser. 



A substage condenser, shown in position in Fig. 10, is, in effect, a 

 large objective with a very high numerical aperture. Its function is to 

 project a wide-angle cone of light through the slide into the objective. 

 As this condenser is working in air, it obviously cannot have an N.A. 

 greater than 1. In the example shown, the full aperture of the condenser 

 is not being used, so that the cone of light entering the objective has 

 an angle of 96°. It follows that the working aperture is 



N.A. = / sin 9 = 1 X sin 48° = 0.8 



Even at the risk of appearing tedious, it must again be emphasized 

 that the objective may have N.A. 0.95 engraved on the barrel, and the 

 condenser may have N.A. 1.4 engraved on the top lens housing. This 

 has nothing to do with the case. The working N.A. is dependent on the 

 actual cone of light being projected, which is in its turn determined by 

 the aperture in the iris under the substage condenser. 



The addition of the wide-angle substage condenser can raise the work- 

 ing N.A. close to, but never beyond, 1. The only way of achieving an 

 N.A. greater than 1, since sin 6 cannot rise above this figure, is to increase 

 /. This is the reason for the use of immersion oil and of immersion lenses 

 made to work in it. This situation is shown in Fig. 11. Immersion oil, 

 with a refractive index of 1.56, has been substituted for air both above 

 and below the slide. This not only increases i but also permits the lenses 

 involved to operate at a greater angular aperture. In this case 



N.A. = i sin d = 1.56 X sin 58° = 1.3 



It should particularly be remarked that if oil is used only between the 

 objective and the slide, the system cannot operate at an N.A. greater 

 than 1. This may not in practice be a bad thing, but this will be dis- 

 cussed later. In the meantime, it is necessary to turn to other require- 

 ments of objectives. 



Magnification. Magnification is, of course, a requirement of an objective 

 even though it is entirely secondary to resolution. The useful limit of 

 magnification, in fact, is that which increases the size of the smallest ob- 

 ject that can be resolved to the smallest object that can be seen. Assum- 

 ing, roughly speaking, that the eye can just see a speck 0.1 mm in 

 diameter, the useful limits of magnification in relation to N.A. of com- 

 mon types of objectives are given in Table 1. 



The total magnification is, of course, a multiplicand of the magnifica- 

 tions of objective and ocular. Even so, it will immediately be seen that 

 the usually employed XlO eyepiece does not result in magnifications 

 anywhere near the useful limit of most lenses. These limits are, however, 



