If we now bring the front lens of an objective 

 close to the cover glass we can study its influence 

 and for this purpose will use the simple but intel- 

 ligible illustrations of Prof. Gage to show the 

 same phenomena. 



I shows a dry objective in which the inter- 

 vening medium between the top of cover glass 

 and front of objective is air. 



II is a water immersion objective in which 

 this space is filled with water. 



III is an oil or homogeneous immersion ob- 

 jective in which the space is filled with oil of 

 cedar. 



From what we have 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 differ- 

 ence 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 homogeneous 

 fluid has the same density as glass, or as we have 

 stated, is virtually fluid glass, no refraction between 

 these two media takes place. 



If we now refer to the diagrams and will bear 

 in mind the course of rays from an object through 

 a cover glass, we can follow out their action. In 

 the case of a dry objective, we see that some of 



