You should never rack downwards with the coarse focusing while looking down a microscope because you could damage the objective lens. The objective lens is the lens that is closest to the specimen, and it is also the most expensive and delicate part of the microscope. If the objective lens comes into contact with the specimen, it can be scratched or broken.

To avoid damaging the objective lens, always rack upwards with the coarse focusing when you are looking down a microscope. This will ensure that the objective lens does not come into contact with the specimen.