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Helmholtz's Reports of Binocular Colour Fusion

Hermann von Helmholtz looked into binocular colour fusion (and the effect referred to here as the "Sky Blue' Observation) in his book "Treatise on Physiological Optics; Volumes II & III".  The relevant sections are in the Optics Text file (which should open in a new browser window).  The following page references are to this text.
    - Binocular Colour Fusion References follow immediately.
    - 'Sky Blue' References 

Binocular Colour Fusion References

Page 505:-  There is a description of fusing the spectrum with a reverse copy of itself.  The result is supposed to be white.  There is also a statement that the reversals are complementary.  A little thought will show these statements to be incorrect.
    First, when the spectrum is reversed, there is a central colour that is the same in both images.  This will not be white when fused.
    Second, colour fusion follows subtractive colour mixing laws for the hue.  The complementary colour pairs that produce white (grey) when fused are red-green, orange-blue and yellow-violet.  Yellow and green are near the middle of the spectrum while red and violet are at the ends.  They cannot overlap their complementaries.

Page 506:-  An equivalent image to the one described by Helmholtz is here.

Helmholtz page 506 image (2692 bytes)


Page 507:-  There may be confusion between binocular colour fusion and paint mixing. Helmholtz states "And so such an observer might possibly be betrayed into thinking that, when he sees red and blue simultaneously, he really does see violet or purple. But if the real compound colour made by the two observed colours is exhibited, the difference between it and the other will be very striking".
    However, binocular fusion results in a brightening of the colour while paint mixtures result in a darkening; therefore they will never be the same. Since Helmholtz was not aware of this difference in value changes, he incorrectly concludes that binocular colour fusion is not a valid colour mixing.  A Bicovi image with the two violets for comparison has been created here.

Helmholtz then arranges prisms of Iceland spar to allow red and blue coloured squares to visually overlap to produce a pink visible to either eye.  The red and blue squares are then visually fused by binocular superposition.  He states that the two results are not the same.  Unfortunately for his powers of observation, the two colours cannot be the same.  The pink is a result of additive colour mixing and binocular colour fusion follows subtractive laws.

Helmholtz page 507 image (3226 bytes)


Page 509:- Two images are described.

Helmholtz page 509a image (3226 bytes)

Helmholtz page 509b Image

Page 512:- When black & white stereograms of differing value are binocularly combined, an impression of lustre is created.  

Helmholtz page 512a Image


Helmholtz page 512b Image


Page 516:- According to Helmholtz, binocular fusion can result in an appearance of transparency.

Helmholtz page 516a Image


Helmholtz page 516b Image


Page 530:- A direct parallel to additive colour mixing is made for binocular colour fusion.  Bicovi colour fusion actually follows subtractive mixing rules.

'Sky Blue' References

Volume III
Page 516:- In less than two pages, the 'Sky Blue' colours are described as:
- Violet for the light-adapted eye
- Blue for the light-adapted eye and Orange-Yellow for the other
- Violet-Grey for the light-adapted eye and Yellow for the other.

Volume II
Page 288:- An explanation is given for the colours being:
- Green for the light-adapted eye and Red for the other.

Helmholtz's records vary from Violet to Green for the light-adapted eye, and Red to Yellow for the shaded eye.  Clearly, there is no consistency.  Some of this inconsistency may be a result of using the concept of complementary colours as an overall organising principle
    As noted in the 'Sky Blue Viewing' section, there is data to support that the colours are Blue and Red.

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