In the early thirties, Dr. Walter Schiller from gynaecological clinics of the university of Vienna observed a thin layer of "cancer-like" cells that did not invade the underlying tissues. He classified this phenomenon as a "superficial cancer of the uterus". Some of his colleagues disagreed: they claimed that however abnormal a group of cells may look, it cannot be described as "cancer", since a cancer is, by definition, invasive. For Schiller, this was an absurd argument. He collected a series of slides that displayed all the intermediary stages of transition from such "superficial cancer" to true cancer, and this for him was enough to prove that one derives from the other:

"(…) there is no morphological similarity between a 10 day old embryo of a mouse and a mouse (…) direct observation is not readily possible either , as we cannot observe the embryo's development into a mouse. The evidence that the embryo is the first link in the development of a newborn mouse can only be secured through a series of microscopic pictures to include the whole evolution with intermediary stages. These changes are not accessible under direct observations, but it is possible to secure a complete series of slides showing intrauterine development. This method, the foundation of embryological research can be applied as well to the development of carcinoma".

Schiller captures one important use of series of slides: the alignment of "stills" that captures a single moment  in order  to represent a developmental unit .  Other series gather samples of "typical specimens" (of plants, of bacteria, of pathological lesions of a given kind).  Such series can be used to create composite pictures and to train the observer to recognize "typical images" and to provide correct classifications or diagnosis.

Other slides may, however, represent an unique specimen. Some slides became examples of a "prefect preparation", praised for its exceptional display of craft and artistic qualities.  Different slides made possible the multiplication of the unique: the same rare specimen, cut into numerous ultra-thin preparations can be circulated among a group of professionals in order to homogenize classifications and to construct consensus. Slides display the full spectrum of use of biological materials: from educational kits to sophisticated research tools. Early microscopes were already sold with boxes of typical "preparations". Schools held series of slides used for teaching and demonstration, as has universities and research institutes. In the latter case, slides typically had a long life, because reclassifications of collections was often a part of ongoing investigation process. Collections of slides were central to production of knowledge in botanic and comparative zoology, in neurology, microbiology and pathology, and numerous other biological and medical domains. They were at the same time depositories of existing knowledge, reference tools, performatives devices and means to diffuse information and homogenise scientific practices. In many domains, circulation of slides defined the limits of scientific communities and the expansion in sues of these objects was one of the elements which favoured the  expansion of  groups of scientists who used them.

Many slides are two-dimensional reductions of three-dimensional objects, and they display a small sample seen as a faithful representation of a much bigger whole. In such cases, scientists often used modelling to construct three-dimensional objects from serial sections. Other slides fix and "prepare" microscopic entities such as blood cells or microorganisms for observation. Such preparations define at the same time the observed object and the correct way to observe it. Other still represent an entire macroscopically visible object (an insect, a flower); such an object is usually fixed, stained and manipulated in order to make it look "life-like".  Studies of techniques of sampling, slicing, fixation and staining of specimens are essential to understand microscopic slides, as are investigations of methods of filing, storage and conservation of collections of slides and the standardisation of slides and cover slips. These techniques reflect the development of microscopy and chemistry, of ways of training the observers' eye and the organization of scientific and medical work.  The juxtaposition of different ways of describing a microscopic preparation: direct observation of specimens, sketches of microscopic preparations, drawings in scientific publications, in atlases and textbooks, photographs, and verbal descriptions illuminated the ways scientific knowledge is generated and diffused, and the role of trained scientific observation and of inscriptions in this process.

In spite of the key role of microscope slides in the production of knowledge in the life sciences, a few historians of science only became interested in techniques of production of microscopic preparations, and even fewer studied collections of slides. There is a rapidly growing body of knowledge on the history of microscopes but much less is known about objects observed under the microscope. This project aims to forward the understanding of microscopic slides as material and epistemic objects, and their place in scientific, medical, museological and visual cultures.