THE IMMERSION SYSTEM 



137 



sufficient magnification can be obtained, while the working 

 distance is greater, the field is larger, and the penetrative 

 power is greater than with the ^ in. lens. 



The immersion system of objectives has many advantages : the 

 loss of light is less, the distance between the cover-glass and the 

 front of the objective the working distance, as it is termed is 

 greater, and more can be seen with an immersion lens than with 



S-. 



FIG. 25. Diagram to illustrate tlje course of rays of light 

 through an objective. 



a dry lens of equal magnifying power. This can be best illustrated 

 by means of two simple diagrams. 



In Fig. 24 let cd represent the surface of a fluid, either water or 

 oil, and let ab be drawn perpendicular to this surface, and cutting 

 it at y. Let ry represent a ray of light proceeding from a rarer 

 medium, such as air, into a denser one, water or oil. As is well 

 known, this ray when it enters either the water or the oil does not 

 continue in the same direction, but is " refracted " or bent nearer 

 the perpendicular ab, the bending being more marked with oil 

 than with water. Thus we may suppose that the direction of the 

 ray in water would be represented by the line yw, and in oil by the 

 dotted line yo. Conversely, a ray of light proceeding from a 

 denser medium into a rarer is bent away from the perpendicular, 

 and the rays wy in water, and oy in oil, would, on emerging into 

 air, proceed in the direction yr. 



In Fig. 25 (which for convenience is drawn somewhat out of 



