Handy squeeze tube dispenser contains 1/2 ounce of this stable optical fluid, which will not dry or harden.
Oil immersion is used in microscopy to increase resolution at higher magnifications. Usually the highest magnification of an objective lens is 100X. The oil prevents light from refracting between the specimen and the objective lens and images are consequently sharper. An oil immersion lens will have some designation on it; look for the designation before immersing any lens.