You need to position the stage to center your object of interest in the field of view. Then it will still be approximately centered when you switch to the oil objective.
If you first focus with the 40x objective you will be almost in focus when you switch to the oil objective.
You need to use a microscope to properly understand this.Why is it important to find good focus using 40x objective lens before you go to oil?
1. Because you have to be able to find the speciman and get it focused before you put it over.
2. Get it centered and ready.
3. Because you cannot go back to the 40x after you put the oil on because it will contaiminate the lense because it directly touches, instead you have to go back to the 10x and try to find it and than go back.
so you know youve got your sample under the lens and it will only need fine focus when you go to oil, on the other hand if you just put the oil on the sampe and wind the platform right up to the lens you can just lower it slowly until it comes into view
Because the specimen will be a lot clearer and easier to find when you switch to the oil immersion lens.
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