refracted away from the axis in the directions b c 

 and b' <? , which are the original directions of the rays. 

 The same action will take place with the extreme rays 

 o e anfl o e' , which will emerge as shown at d f and 

 d' f . If the front lens of an objective is brought 

 close to the cover glass we can study its influence and 

 for this purpose will use the simple but intelligible 

 illustrations of Prof. Gage, Fig. 36. 



I shows a dry objective in which the intervening 

 medium between the top of the cover glass and the 

 front of the objective is air. 



II is a water immersion objective in which this 

 space is filled with water. 



III is an oil or homogeneous immersion objective 

 in which the space is filled with oil of cedar. 



From what has been said in regard to the laws of 

 refraction we know that as the medium becomes more 

 dense the refraction becomes greater and for the same 

 reason it is clear that as the difference in density 

 between the two media becomes less the refraction is 

 proportionately less. As water has a greater density 

 than air, but less than glass, refraction between 

 these two media is less than between air and glass. 

 As the oil of cedar has the same density as glass, the 

 oil and glass are virtually a homogeneous mass and 

 no refraction between these two media takes place. 



75 



