Seeing and telling the invisible: problems of a new epistemic category in the second half of the eighteenth century

ABSTRACT The invisible object, in the eighteenth century, is not an evidence. It is the result of textual and semantic learning. Which concrete strategies are used to construct and depict objects out of sight? How do we make them a cognitive reality acceptable to a scientific community? This paper first highlights the conditions for the emergence of a field of microscopic knowledge and its epistemological consequences. Then we consider the microscopic gaze in terms of learning, situated between the act of observation as such and discursive practices. We conclude by studying a concrete case of “negotiation of the invisible” in a correspondence between Carl Linnaeus and John Ellis concerning corpuscles observed in mushroom infusions.

following the discovery of freshwater polyps by Abraham Trembley. 6Moreover, the role of microscopic research carried out as part of the experimental program on spontaneous generation was central in the eighteenth century in the development of the disciplinary field of the invisible.It was during this period that one of the major problems associated with microscopy received its first satisfactory solution: the establishment of a classification of animalcules which would make it possible to stabilize the system of reference and communication about microscopic bodies. 7Thematizing and solving partially the problems of communication (description, nomenclature and classification) in the eighteenth century made it possible to lay the foundations of microbiology in the nineteenth century.A first formalization which applies nomenclature rules to a logic of classification according to standard normsin Latin language according to Linnaeus's modelwas established by Otto-Friedrich Müller as early as 1773. 8e can, therefore, take the position of approaching the history of microscopy as an exemplar of the history of scientific communication.On the basis of observational accounts in periodicals such as the Journal des sçavans and the Philosophical Transactions, Marian Fournier has underlined the absence of an immediate correlation between technical innovations and the development of microscopy as a field of research. 9Material improvements mean, for the scientists, relearning to see through new lenses and readapting their way of communicating accordingly.Microscopic vision and the discourses are therefore not self-evident; they are the result of textual and semantic learning as well as codification that can be modified by new ways of seeing.Which concrete strategies are used to construct and depict objects out of sight?How do we make them a cognitive reality acceptable to a scientific community?To show how crucial these questions were in the eighteenth century, I will first highlight the conditions for the emergence of a field of microscopic knowledge and its epistemological consequences.I will then consider the microscopic gaze in terms of learning, situated between the act of observation as such and discursive practices, and will conclude by studying a concrete case of "negotiation of the invisible" in a correspondence between Carl Linnaeus and John Ellis concerning corpuscles observed in mushroom infusions.

Building the invisible
The invisible, as an object of investigation, is not self-evident.Putting the eye to the microscope, observing everything that separates spontaneously perceived reality from that which appears under magnifying glasses, is not enough to clearly define a field of research.Scientists only gradually identify the epistemological consequences of the first discoveries.Marc Ratcliff, in The Quest for the Invisible, outlines three main stages in the construction of what he calls the "true invisible". 10) The first one extends from the 1680s to about 1740: microscopic objects are not systematically invisible to the naked eye, this category of body being far too controversial and dependent, to be perceived, on the quality of microscopes.Insects, worms, seeds and other small organisms are the favorite objects of investigation.The truly invisible ones (Leeuwenhoek animalcules, in particular) are engaged during this period in what Alexandre Métraux defines as a process of "social normalization", 11 which will lead to the recognition of their existence.(b) In 1740, the second period begins with the discovery of the fresh-water polyp by Abraham Trembley.Nature then reveals unsuspected potentialities; the limits between the three kingdoms of nature become unclear, and the aquatic environment is chosen as the space of observation par excellence.New conditions are thus created to shift the attention to the invisible, to gradually start looking for particular organisms escaping the common vision.Their status, however, remains unclear: organic molecules, animalcules, plant-animals, zoophytes, Linnaean "chaos" are all terms under which the fascination for this space of a new knowledge can be identified.This period gave rise to numerous interpretative questions, leading to a renewal of the controversy over spontaneous generation, notably around the observations of John Tuberville Needham.12 At the same time, classifications were established, developing into the first real systematic of animalcules.13 (c) From this point on, the relationship to infusoria changed radically: from then on, the scientist worked on the basis of a framework of determination, classification and nomenclature that should allow each naturalist applying the Linnaean canons to identify organisms in the same way.The solution brought by systematics to the problem of how to "repeat" the organism, i.e. to give a determination with a generalizable validity, appears then as one of the important factors for the creation of a category of "truly invisible" corpuscles, and the progressive constitution of a naturalist community around the infusoria, between 1780 and 1830.
In the context of the scientific thought of the eighteenth century, dominated by the notion of visibility, 14 this emergence of a field of exploration of nature based on extremely tenuous beings, invisible to the naked eye in their entirety or in their essential properties, led to two consequences: on the one hand, a major upheaval in the general representation of nature, which suddenly extended to unsuspected worlds (Lamarck noted that they were undoubtedly much more populated than the visible one 15 ); on the other hand, they raised fundamental questions about the economy of life. 16urther, a great epistemological question emerged about the link between visibility and knowledge. 17The invisible thus becomes an epistemic category, a problem 18 ; this category no longer applies only to infusoria and animalcules, but also allows the systematic exploration of structures and corpuscles inaccessible to the naked eye.However, depending on whether we consider it from a classificatory or experimental point of view, this new field of research led to very different types of discourses and problems.From a linguistic point of view, the Linnaean-inspired nomenclature will, as we know, play on a drastic restriction of utterances, towards a neutralization of expression, allowing pragmatic communication.This is reflected in the minimal definition that allows it to integrate a standardized knowledge system. 19The experimenters also observe with the aim of naming and classifying, but the gesture that places the invisible object in the field of the microscope leads the scientist to attempt a first understanding, to question it, possibly later to note unusual, incomprehensible elements or "anomalies" (extraordinary reproductions, in particular, or "resurrections" 20 ), which should lead to the implementation of specific devices.On the discursive level, the process involved is much more complex: notation can first be seen as a "personal" form of presentation by the scholar, an act of understanding; it will then be reworked in the perspective of various types of dissemination and exchange such as correspondence, communications to academies and publications.Taking notes, looking for the right words, becomes the tool of a long process of envisioning, in the sense that Ohad Parnes gives to the term: "To envision indicates not simply to visualize, but also to envisage, to apply scientific mental frames and epistemological categories". 21oth the classificatory operation and the experimental approach lead to controversy, discussions and exchanges that gradually contribute to build an "acceptable" discourse for the scientific community.But experimentation differs from taxonomy in the epistemological, philosophical and metaphysical consequences that some observations open up.At the beginning of microscopy, hypotheses on spontaneous generation, for example, replace superficial or technically limited observations. 22In the 1750s, they are updated thanks to the new capacities of scientists to penetrate structures, to observe behavior and to focus their gaze on the truly invisible world.Thus, even if the classificatory operation is being perfected, even if natural history seems to be moving towards the definition of a specialized knowledge of invisible beings, the problems relating to their properties and to their meaning are deepening in the experimental space.When Linnaeus chooses to create, within the genus CHAOS, the species of Redivivium, he transforms the capacity of animalcules to resuscitate into a property.This gesture can be understood in two ways: either as a neutralization of what contemporary experimental science is constantly problematizing 23 or as a designation of new questions in the space of a new science.

Microscopic vision and language
I have deliberately used the term meaning above.It refers to the idea that the observation of new entities cannot take place outside of a conditioned semantic space, whether we consider it with Kuhn as a "theory" guiding the eye 24 or, in a more nuanced and complex perspective, as the set of data that inscribes an individual's point of view in a given context.Ludwik Fleck evokes traditions, proto ideas, the form of knowledge acquired and the "style of thinking" imposed by a given community. 25But what happens when a scholar, in the mid-eighteenth century, first observes a body without knowing what it is, and without the possibility to "share" it with others?Clearly, the act of seeing, as well as the account of it, will be highly conditioned by ambient knowledge, both when it leads to errors of perception and when it gives rise to attested and innovative observations.When it comes to seeing and understanding the invisible, however, the knowledge of the time is a non-knowledge.It lacks a specific framework and this absence can only be filled using terms that can structure a new imagination.Logical and analogical connections create this framework and guide future investigations.The specific language used to describe these new "things" cannot escape the subjectivity of the observer, 26 and this is of particular interest to me: if the theoretical, methodological and communicational norms of natural sciences are already partially established, 27 they seem inadequate when it comes to dealing with the invisible.How can knowledge about objects that are so difficult to share be put into practice?How to create a community of understanding and opinion around them? How to answer the philosophical questions they raise, without common representations and observations validated by the community?
These questions not only involve rhetorical and discursive problems, but also concrete problems of perception.In other words, faced with new worlds, naturalists see, physiologically speaking, but this seeing can only become relevant once the means to learn to see and speak have been put in place.It will be necessary to construct, through the practice of observation and the exchanges to which it gives rise, an adequate conceptual and discursive space to grasp and express the new reality.In concrete terms, the first step is to bring the new or problematic object closer to a known and controllable space: this is what Trembley, for example, does in the case of the polyp. 28Once the identity of the object has been accepted, the scientist's eyes can suddenly be opened to the properties of other productions hitherto ignored or wrongly perceived.
Throughout this process, the link between the language used to take charge of the observation and its meaning is obviously crucial: the choice of terms places the being in a semantic and pragmatic space, which then acts as a filter during the act of observation.As Hanson has clearly shown, 29 the relationship between vision and knowledge cannot be limited to perception alone.It rests on the logical categories that the observer uses to account for what he sees. 30The categories that are active in the immediate context of the scholarideologies, the intellectual sphere, the institutional community, etc.therefore not only influence individual perception by directing the observation to the salient features of nature; they also affect the way of reporting on them in a shared language. 31Different phases can be distinguished here.
(a) An elaboration phase, attested by numerous notes, laboratory notebooks, where the observation is first formalized.Whether they are intended to be transformed into communication or not, 32 these notes bear witness to attempts to understand the object, to terminological hesitations, to the establishment in language of an initial "network of meanings" 33 and to a gradual change in the scholar's understanding of the object.(b) A communication phase that, in general, is primarily concerned with person-toperson exchanges.As we will see in the third part of this article, adjustments take place here both at the level of the observer, who recasts his notes into a more coherent discourse, and in the exchange itself.(c) A publication phase, either through public correspondence and academic papers or in the form of a treatise.Here again, a new re-elaboration shows an adaptation of the discourse and, above all, a willingness to fix the representation.This opens up a space for controversy that will lead to new observations, reformulations, representations, etc.
These different moments obviously do not follow a purely linear logic.A complex game of back and forth is set up between the knowledge being elaborated, the shared knowledge and the new elements revealed by the observations and the hypotheses to which it gives rise.At every levelindividual, interpersonal, community and public receptiona long process of multiple negotiations will gradually give the invisible an existence and a meaning. 34'm not able rightly to understand … " To illustrate this idea of negotiation, I have chosen to focus on an exchange between two great European scientists of the time, John Ellis and Carl Linnaeus, about the nature of certain corpuscles they observe in mushroom infusions.This took place in 1767 and 1768.Linnaeus published the twelfth edition of his Systema naturae in 1767.In the genus CHAOS, he refers to the species fungorum (literally: "[from] mushrooms") as spores that come alive on contact with water and then turn into mushrooms when they die.These "seeds", Linnaeus adds, are the counterpart of zoophytes: while zoophytes testify to the metamorphosis of the plant into an animal, fungorums are animalcules that become plants. 35This transmutationist vision is not new.It is notably supported by John Tuberville Needham, observing infusions of supposedly sterile plant substances that gave rise to the appearance of plant filaments, then animalcules. 36It shows the aporia of an experimental science which, when it comes to microscopy, never manages to touch the causes of phenomena: observing the mechanisms of the emergence of life in their precise sequences remains impossible.
Linnaeus therefore submitted his observations to John Ellis and asked him to reproduce and verify them. 37All the discussion focused on this hypothetic passage from the vegetal to the animal, refuted by Ellis.Although the two correspondents did not use the same language -Linnaeus wrote in Latin, Ellis in Englishthe disagreements went far beyond mere linguistic misunderstandings.From the outset, in his first response to Linnaeus about his observations, Ellis focuses on issues of technical and methodological competence: I have lately been trying experiments on the seeds of the Fungus, called by you Agaricus campestris; and also on those called the Agaricus fimetarius.The minuteness of these bodies, obliged me to make use of the first magnifying glasses in the microscope.This plainly shewed to me, that these seeds, though put into water according to your directions, have no animal life of their own, and are only moved about by the animalcula infusoria, which give them such a variety of directions, both circular, as well as backward and forward that they appear as if alive.The animalcula are so numerous, and at the same time so pellucid, that without good glasses the most accurate observer may be mistaken. 38thout the support of any image, because his correspondent can potentially see the same scene, Ellis gives some purely technical advice, apparently, which is in reality a way of guiding Linnaeus's observations: one must have precise equipment and know how to focus on beings which, by their smallness and transparency, can be confused with the agitation of water.The Swedish scientist is aware of the superiority of his interlocutor's observational skills. 39He will try to misinterpret his perspective not on this particular point, but on the use of language: I received yours, in which you speak of the living seeds of Fungi, asserting that you have only seen the animalcula infusoria moving the powder of these vegetables.I am not able rightly to understand whether you have actually seen the animalcula or not. 40r Linnaeus, if they are the same animalcula, they should gradually settle at the bottom of the glass and then turn into fungi.He believes he can identify them as "the living Seeds of Mould, Mucor". 41The naturalist, therefore, postulating a passage both between kingdoms and between genera, closes his reflection with a clever formula: while implicitly acknowledging how controversial the hypothesis is, he urges his correspondent to confirm it, flattering the latter about his exceptional perceptive abilities: before I venture to put forth such an opinion, I beg of you to lend me your lynx-like eyes; and you will see in the vessel or glass … whether these bodies do not change to plants of Mucor. 42he metaphysical implication of such a statement does not leave Ellis indifferent.Taking up the very rhetorical technique of Linnaeus, he will first point out an uncertainty about the clarity of the message itself: "By your letter, you seem to think, that the seeds of the Fungi are animated, or have animal life, and move about". 43The uncertainty is underlined both by the modalization of the verb ("you seem to think"), and by the disjunction ("are animated, or have animal life").Ellis also plays on formulations: when he quotes Linnaeus, he uses this "or" as a signal of imprecise interpretation (the expression "are animated, or have animal life" does not necessarily have to be understood in the active sense of the term).Later in his letter, even though he admits to facing phenomena that are difficult to analyze, he shows how important it is to use language with precision, and to mark the site of the problems: I have now a Lycoperdon Bovista, which I received from our good friend P. Collinson four days ago.I put part of it into river water, and in two day's time, I perceived the seeds or farina of it moving about distinctly.The fourth day I perceived the figure of the animalcula that moved them.Are these seeds, or these animalcula, (for they are evidently distinct bodies) to turn into Fungi, Mucores or Lycoperda?… If the animalcula that moved the seeds of the Lycoperdon, it would be amazing; and again, it would be as surprising that the seeds of one genus should produce another; for instance, that the seeds of Lycoperda should produce Mucores.However I have determined to go through these experiments with precision, and to call in witnesses of the several appearances. 44e expert can't be confused by appearances: he brings to light everything that may seem incomprehensible and carries out a systematic interrogation based on the confrontation of several observations.In the absence of more precise testimonies, the idea of animalcules that can "grow into Fungi" does not mean nothing. 45For Ellis, it can only be a question of animalcules initially existing in the water in the form of eggs or germs, awakened by the presence of fungus dust, then feeding the mold on their remains after their death.In this sense, and only in this sense, the visions can be connected: "If you mean that animalia infusoria, when they are dead, are a proper pabulum for Mucor, I agree with you; for I have many animal substances that are covered with Mucor". 46innaeus read these experimental interrogations without really taking into account all of Ellis's precautions.On the contrary, he only remembered from the above-mentioned relationship the fabulous appearance of animalcules apparently coming from the spores. 47vidently losing his patience, Ellis copies in his answer the draft of his former letter: "I thought it necessary to quote from my former letter of 30 th October, as my real opinion". 48He quotes from his laboratory journal too, and relies on external and prestigious testimonies -Daniel Solander in particularto put an end to the quarrel: it appeared evendently to him [Solander], and many more gentlemen who saw my experiments, that the motion which they had, proceeded from animalia infusoria, whose shape we plainly saw and observed distinctly the particular motion, with some attention, which these little creatures had while they were eating the seeds of the Fungi, and which they communicated to the seeds of the Fungi, so as to make them appear alive. 49

Conclusion
This example is representative of the way in which microscopic vision engaged in the eighteenth century in a process of definition that went far beyond the technical issues.
Learning to see, learning to make others see, is as much a matter of words and language as of observation.The problems relating to the implications of specialized vision are often thematized; this shows that scientists are aware of the importance of the observer in the process of observation.Certain productions seem to be impossible to approach without engaging, at the very moment of perception, many presuppositions that will contribute to imposing a meaning.This long period of negotiation around invisible beings participates centrally not only in the emergence of a "microbiology", but also in a global redefinition of the conditions of knowledge production, between empiricism and theory.