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Tag Archives: Experimentum Crucis

Newton’s ‘Crucial Experiment’

Kirsten Walsh writes…

In his first optical paper, Newton claims that he has performed an Experimentum Crucis, which proves that refrangibility is an original property of the light, not an effect of the prism:

    …the true cause of the length of that Image was detected to be no other, then that Light consists of Rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

This experiment and its role in Newton’s theory of colours raises some questions that I’m not really sure how to answer.  I hope you can help me.

Firstly, let’s have a closer look at this Experimentum Crucis:

Newton's Experimentum Crucis

White light travels from the Sun (S), through the first aperture (F), through the first prism (ABC), where it is refracted for the first time, producing an image on the first board (DE).  A small amount of light passes through the second aperture (G), producing an image on the second board (de).  A small amount of light passes through the third aperture (g), through the prism (abc), where it is refracted for the second time, producing an image on the screen (MN).  Newton “took the first Prisme in [his] hand, and turned it to and fro slowly about its Axis”, so that different parts of the refracted image could pass through the apertures to the second prism.  He took careful note of where each image appeared on the board MN.

Newton finds that each time a particular ray passes through a prism it refracts to precisely the same degree.  For example, light that refracts to 50 degrees at the first prism refracts to 50 degrees at the second prism as well.  Newton argues that this shows that refrangibility is an original and constant property of light.

Newton’s Experimentum Crucis was heavily criticised by his contemporaries.  Hooke, for example, argued that this experiment is not a crucial experiment, because it does not prove that colour is an original property of light.  Hooke believes that light becomes coloured as it passes through the prism, and Newton’s experiment does not convince him otherwise.

While colour is conspicuously absent from Newton’s discussion of this experiment, this line of criticism is extremely common.  For example, Newton’s contemporaries, Hooke, Huygens and Pardies, and more recently, writers such as Sabra and Bechler have all made criticisms along these lines.  As I have previously discussed, Newton used mathematics and measurement in order to achieve absolute certainty.  So it is no accident that Newton only discusses refrangibility and not colour in this experiment.

Newton concludes that white light is composed of rays of every colour in equal amounts, but he argues for this in two steps:

1)      Light is a “Heterogeneous mixture of differently refrangible Rays”; and

2)      There is a one-to-one correspondence between refrangibility and colour.

So, while the Experimentum Crucis only supports step (1), it is often mistaken as an argument for Newton’s conclusion.  Newton takes a great deal of care to establish (1) experimentally, but he seems to take little care at all to establish (2), and hence, the conclusion.  In his first optical paper he simply asserts it as proposition 2; in his reply to Huygens he asserts it as a note to his definitions.

This raises two questions.  Why did Newton take so little care over step (2)?  How did Newton’s main opponents miss this lack of care?