104 ELEMENTARY TECHNOLOGY. 



represented as spread out by refraction through the cover glass 

 so that the rays from the object C pass through the edge of 

 the lens B as if the focus was at E, while the more central 

 rays have their apparent focus at D. Hence, with a thick 

 cover glass there must be imperfect definition. For high 

 powers the thinner the cover the better. Optician's sizes known 

 as 1,2, 3. If there were no cover glass all the rays would di- 

 verge from C, and the objective would require to be Aplanatic 

 (free from error), or having all the rays brought to the same 

 focus. But since with a cover glass the marginal rays appar- 

 ently diverge from a nearer focus than the central rays, the 

 objective will only define well when it is what is called under 

 corrected. The increased curve of the convex or crown lens 

 renders the flint lens unable to neutralize all the spherical aber- 

 ration. An opposite condition is termed over corrected. A 

 flatter crown lens and a deeper concave flint cause the marginal 

 rays to have a longer focus than the central ones. An aplanatic 

 objective can be made into an under-corrected one by causing 

 the back lenses to approach the front lens. For this purpose a 

 screw collar is provided in the objective which moves the lenses 

 to and fro. It is better it should move the back lenses than the 

 front. Another way of obtaining the same result is to cause the 

 eye piece to vary in distance from the objective by means of a 

 draw tube. Such arrangements are needed in high power 

 work to compensate for thickness of cover glass, and also for 

 varying tube lengths of different opticians. 



6. Dry and immersion objectives. 



An oblique ray from the cover may pass away from the ob- 

 jective, while in water, whose refractive index is higher than air, 

 it is bent towards the lens, as in Fig. 7, PI. i. In homogene- 

 ous immersion objectives, where oil of cedar, or thickened 

 glycerine, etc., are interposed, still more rays are utilized. 

 This light-grasping power is expressed by the term Numeri- 

 cal Aperture. 



7. Numerical aperture. 



Numerical aperture must not be confounded with the older 

 term, angle of aperture, which merely expresses the relation 



