Q: When I read in bed I鈥檝e noticed that, when lying on my left side, the colour of the page when viewed with my left eye appears to be pink compared with the same page viewed with my right eye. If I turn on to my right side, the situation is reversed. What is the reason for this?
A: The phenomenon suggests blood pooling in retinal venules. There is generalised relaxation of the venous system when you are warm and lying in bed, which is why you may feel faint if you get out of bed quickly. In this case gravity causes pooling of the blood in the veins of your legs and abdomen, leading to a fall in blood pressure and consequently in blood flow to the brain.
Lying with one eye lower than the other, gravitational pooling of blood in the venules of the lower eye would provide an out-of-focus film of redness through which external images would be viewed.
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A: It might be tempting to suggest that pooling of blood on the side on which you are lying colours your vision in that eye.
More likely, it is your pillow that holds the answer. Because your head sinks slightly into the pillow the visual field of the eye on the side on which you are lying is reduced. As light reflected off the page enters the eye, red light rays are refracted less than blue. The small difference in refraction for red and blue light results in a reddish image being focused on the fovea centralis in the retina, and the observer sees a pink page.
This chromatic aberration is not usually noticeable but becomes apparent here because of the partial line of sight and reduced illumination of the eye in question.
Readers may like to investigate which of these two effects contributes more to the illusion by performing the obvious experiment 鈥� Ed.