To examine bacteria and other very small objects, we will need to use the oil immersion (100X lens). Special techniques must be used when using the 100X objective. With the 10 fold increase in magnification provided by the ocular lens, we are approaching the theoretical limit of light microscopy so must obtain the best possible resolution in order to get a clear image the sample. Because light travels more slowly through glass and most other media than it does through air, light leaving air and passing though glass is bent. You have probably observed this bending phenomenon when looking at objects in water whose position and appearance seem distorted. To prevent this distortion from occurring between the sample and the objective lens, we fill the space between with immersion oil which has the same optical properties as glass. Without immersion oil, objects seen under the 100X lens will appear fuzzy even at the best possible focus.