Chapter 3 - Figure 3.3 - Refraction in the compound microscope using an oil immersion objective lens - Question - Page 59: 1

It is necessary to keep the light rays from refracting.

While this is not a problem at lower levels, and magnifications over 900 times, it is necessary to consider how the light rays will refract. By adding oil, which has the same refractive index as glass, this problem is solved.