I believe it's the light button, but it appears it needs a new battery. Comes with the slide.

This Project Mc2 Ultimate Lab Science Experiment Kit Replacement Parts Microscope is a must-have for any aspiring scientist. With its high-quality design and advanced features, this microscope is perfect for exploring the world of science and nature. The microscope is brand new and has never been used, ensuring that you receive a product that is in perfect condition.

The microscope was manufactured in China and comes from the popular Project Mc2 brand. It is suitable for children aged 8 and above and is a great educational toy for anyone interested in science and nature. The microscope is easy to use and comes with a range of features that make it perfect for exploring a variety of different specimens.