Answer;
Low power lens
Explanation;
-When focusing on a slide, it is important to always start with low power objective lens (4X or 10X). The 4x objective lens has the lowest power and thus the highest field of view. As a result, it is easier to locate the specimen on the slide than when you start with a higher objective lens.
-Once you have the object in focus, then switch to the next higher power objective. One should then focus again on the image and then switch to the next highest power such as 25x, 40x, etc.