These may be damaged if they hit the specimen. T he magnification is indicated by a number on each objective. Furthermore there is a progression in size such that the longest objectives have greatest magnification. Note the black line near the tip of the objective. This is used to identify an oil immersion lens.
P lace a small drop of oil on the objective lens.
A small drop of oil must also be placed on the cover slip. The lens should be carefully lowered into the oil prior to focusing. O bserve this with your naked eyes focusing on the objective and the specimen. Do NOT look through the oculars. Oil improves the optics because it unites the glass cover slip and the objective.