Step-by-step explanation:
For anything to be seen it must either emit light or reflect light. When you look at the functioning of a microscope you will notice that when you put specimen under the microscope, you need an external light source to illuminate the specimen. For this purpose a light source is placed right under the specimen holder.
For the specimen to be visible clearly the slide is made transparent so that it is backlit by the light and easily visible in the microscope.