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Tag Archives: Opticks

Observation and Experiment in the Opticks: A Baconian Interpretation

Kirsten Walsh writes…

In a recent post, I considered Newton’s use of observation and experiment in the Opticks.  I suggested that there is a functional (rather than semantic) difference between Newton’s ‘experiments’ and ‘observations’.  Although both observations and experiments were reports of observations involving intervention on target systems and manipulation of independent variables, experiments offered individual, and crucial, support for particular propositions, whereas observations only supported propositions collectively.

At the end of the post, I suggested that, if we view them as complex, open ended series’ of experiments, the observations of books 2 and 3 look a lot like what Bacon called ‘experientia literata’, the method by which natural histories were supposed to be generated.  In this post, I’ll discuss this suggestion in more detail, following Dana Jalobeanu’s recent work on Bacon’s Latin natural histories and the art of ‘experientia literata’.

The ‘Latin natural histories’ were Bacon’s works of natural history, as opposed to his works about natural history.  A notable feature of Bacon’s Latin natural histories is that they were produced from relatively few ‘core experiments’.  By varying these core experiments, Bacon generated new cases, observations and facts.  The method by which this generation occurs is called the art of ‘experientia literata’.   Experientia literata (often referred to as ‘learned experience’) was a late addition to Bacon’s program, developed in De Augmentis scientiarum (1623).  It is a tool or technique for guiding the intellect.  By following this method, discoveries will be made, not by chance, but by moving from one experiment to the next in a guided, systematic way.

The following features were typical of the experientia literata:

  1. The series of observations was built around a few core experiments;
  2. New observations were generated by the systematic variation of experimental parameters;
  3. The variation could continue indefinitely, so the observation sequence was open-ended;
  4. The experimental process itself could reveal things about the phenomena, beyond what was revealed by a collection of facts;
  5. The trajectory of the experimental series was towards increasingly general facts about the phenomena; and
  6. The results of the observations were collated and presented as tables.  These constituted the ‘experimental facts’ to be explained.

Now let’s turn to Newton’s observations.  For the sake of brevity, my discussion will focus on the observations in book 2 part I of the Opticks, but most of these features are also found in the observations of book 2 part IV, and in book 3 part I.

Figure 1 (Opticks, book 2 part I)

The Opticks book 2 concerned the phenomenon now known as ‘Newton’s Rings’: the coloured rings produced by a thin film of air or water compressed between two glasses.  Part I consisted of twenty-four observations.  Observation 1 was relatively simple: Newton pressed together two prisms, and noticed that, at the point where the two prisms touched, there was a transparent spot.  The next couple of observations were variations on that first one: Newton rotated the prisms and noticed that coloured rings became visible when the incident rays hit the prisms at a particular angle.  Newton progressed, step-by-step, from prisms to convex lenses, and then to bubbles and thin plates of glass.  He varied the amount, colour and angle of the incident light, and the angle of observation.  The result was a detailed, but open ended, survey of the phenomena.  Part II consisted of tables that contained the results of part I.  These constituted the experimental facts to be explained in propositions in part III.  In part IV, Newton described a new set of observations, which built on the discussions of propositions from part III.

When we consider Newton’s observations alongside Bacon’s experientia literata, we notice some common features.

Firstly, the series of observations was built around the core experiment involving pressing together two prisms to observe the rings that appeared.

Secondly, new observations were generated by the variation of experimental parameters: i.e. new observations were generated, first by varying the obliquity of the incident rays, then by varying the glass instruments, then by varying the colour of the incident light, and so on.

Thirdly, the sequence of observations was open-ended.  Newton could have extended the sequence by varying the medium, or some other experimental parameter.  Moreover, at the end of the sequence, Newton noted further variations to be carried out by others, which might yield new or more precise observations.

Fourthly, the experimental process itself revealed things about the phenomenon, beyond what was revealed by a collection of facts.  For example, in observation 1, Newton noticed that increasing the pressure on the two prisms produced a transparent spot.  The process of varying the pressure, and hence the thickness of the film of air between the two prisms, suggested to Newton a way of learning more about the phenomenon of thin plates.  He realised he could quantify the phenomenon by introducing regularly curved object glasses, which would make the variation in thickness regular, and hence, calculable.

Fifthly, the trajectory of the experimental series was towards increasingly general facts about the phenomenon.  Newton began by simply counting the number of rings and describing the sequence of colours under specific experimental parameters.  But eventually he showed that the number of rings and their colours was a function of the thickness and density of the film.  Thus, he was able to give a much broader account of the phenomenon.

Finally, these general results were collated and presented as tables in part II.  Thus, the tables in part II constituted the facts to be explained by propositions in part III.

Many commentators have emphasised the ways that Newton deviated from Baconian method.  However, when viewed in this light, book 2 of the Opticks provides a striking example of conformity to the Baconian method of natural history: Newton led the reader from observations in part I, to tables of facts in part II, to propositions in part III.  Moreover, it ended with a further series of observations in part IV, emphasising the open-endedness of the art of experientia literata.

In contrast to the observations in book 2, Newton’s experiments in book 1 look like Bacon’s ‘instances of special power’, which are particularly illuminating cases introduced to provide support for specific propositions.  I’ll discuss this next time.  For now, I’d like to hear what our readers think of my Baconian interpretation of Newton’s observations.

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