It’s necessary to use oil for 100X lens to observe microbial because each objective has different distance, the actual distance (in millimetres or microns) between the front lens of an objective and the surface of the coverslip is known as the ‘Working Distance’. An objective with 10X magnification has an approximate working distance of 4 mm, whereas a 100X oil immersion objective has a working distance of approximately 130 microns that nearly with the coverslip. So, we necessary to use oil for 100X lens to observe microbial that reduce the objective hit the coverslip.