Objects may appear upside down and backwards under some microscopes due to the type of lens being used. Convex lenses, those that curve outward, converge light rays, making objects appear upside down and reversed. However, not all microscopes alter images in this way.
Most microscopes are compound microscopes; they use two or more objective lenses and an eyepiece lens or two to magnify objects. The objective lenses are closest to the object being studied and can be increased in magnification by rotating different strengthened lenses into place directly over the object. The ocular lenses are closest to the eye. Both types of lenses are convex; they are curved outward. This makes light rays converge upon a central point, which is also known as the focal point.
While convex lenses make objects appear reversed, the images can be corrected through the use of a viewing screen or by studying computer images of the slides. However, images that are upside down and reversed can be studied just as easily if the viewer takes into consideration the altered state of the images and manipulates the slides under the microscope in a way that allows him to view and keep track of certain parts of the image.