This sounds a lot like how an oil is used to look at slides in medical labs. For example, it may be standard procedure to smear a sample along the slide, place

This sounds a lot like how an oil is used to look at slides in medical labs. For example, it may be standard procedure to smear a sample along the slide, place it on a stainer, retrieve it later, put some oil on it, and the lens of the actual microscope will come in contact with the oil.