Histology mounts are often what comes to mind for many people when "old microscope slides" are mentioned. Without doubt, our knowledge of the human body's structures and processes, as well of those of animals and plants,  owes a great deal to the early slide makers of the Victorian era. Before about 1850, most mounted histology specimens were thick sections of tissue, sometimes with the vascular structures injected with various coloured substances to make them more visible, and usually mounted as "deep cell" fluid mounts (see top row, 2nd from left - engraved "Skin from the Ear of the Cat showing the Vessels of the bulbs, & the hairs   Hett, 1849" ). These thick preparations could only be examined microscopically with reflected (incident) top lighting, and provided limited information. The early commercial slide preparers such as C. M. Topping, J. T. Norman, A. C. Cole, and E. Wheeler  often worked closely with leading medical professionals of the era to further current research and knowledge in the fields of medicine and surgery. These individuals, by their dedicated efforts, helped develop the techniques in preparation and preservation that allowed the research underlying the remarkable advances in medicine that we benefit from today. An important early development was consistent thin sectioning of tissue specimens, and preparation/preservation techniques that enabled use of transmitted (through the specimen) lighting. This advance allowed microscopic examination of the minute structures, helping open the door to a better understanding of the processes of disease, and the development of new effective treatments. A wide variety of histology mounts were prepared by most of the more capable preparers, covering all the major tissue types (both healthy and diseased) of both human and animal species.

Injected and corroded kidney section showing structures (combination transmitted and reflected lighting). 

Photomicrograph (DIC) of Section of Human Tongue from antique slide (transparent injection by Topping c. 1860s) showing the papillae. Large structure in central area of image is a fungiform papillae (location of the actual taste buds), while elongated "feathery" structures to the left are filiform papillae. These types of views were not possible before competent thin sectioning.