A basic laboratory microscope passes light through a specimen mounted on a glass slide. The specimen needs to be very thin to allow the light to pass through powerfully and without distortion and to allow a cover slip to sit evenly over the specimen.
A standard microscope is designed to show detail in one dimension, and so a specimen does not need to be thick. A different system must be used to look at 3-D detail.
For an image to be seen clearly through a microscope, light must pass through the specimen with high power. Light cannot pass powerfully through a specimen that is more than a few cell layers thick.
Together a typical glass slide and cover slip are about 1 to 1.4mm thick. Microscopes are calibrated to compensate for light bending as it passes through them. As a result, a thick specimen will throw off the adjustment and distort the image.
At higher magnification, very little space is between the lens and the slide. A thick specimen can be smashed by the lens.
It is not a good idea to try to use a thicker specimen without a cover slip. The cover slip is critical to correct the bend of light, especially if magnification of 20x or more is used.