MOTION MOUNTAIN

178 4 images and the eye – optics
F I G U R E 128 A limitation of the eye (see text).
Challenge 178 s
Challenge 179 s
Do we see what exists?
Sometimes we see less than there is. Close your left eye, look at the white spot in Figure
128, bring the page slowly towards your right eye, and pay attention to the middle
lines.Atadistanceofabout15to20 cmthemiddle linewill seemuninterrupted. Why?
Look with oneeye at a full computer screenthat is blinking blue andblack, at a rate
Ref. 132 ofonceortwiceasecond. Nowlookat thesameblinking screenthroughablue filter (a
BalzersK45oraKodakBG12filter).Youwill seeaspot.Why?
Sometimes we see more than there is, as Figures 129 and 130 show. The first figure
showsthatparallellinescanlookskewed,andthesecondshowaso-calledHermannlattice,
named after its discoverer.* The Hermann lattice of Figure 130, discovered by Elke
Lingelbach in 1995, is especially striking. Variations of these lattices are now used tounderstandthemechanismsatthebasisofhuman
vision. For example, they can be used to
Ref. 133
determine how many light sensitive cells in the retina are united to one signal pathway
towards the brain. The illusions are angle dependent because this number is also angle
dependent.
Our eyes also ‘see’ things differently: the retina sees an inverted image of the world.
There is a simple method to show this, due to Helmholtz.** You need only a needle and
a piece of paper, e.g. this page of text. Use the needle to make two holes inside the two
letters ‘oo’. Then keep the page as close to your eye as possible, look through the two
holes towards the wall, keeping the needle vertical, a few centimetres behind the paper.
You will see two images of the needle. If you now cover theleft hole with your finger, the
* Ludimar Hermann (b. 1838 Berlin, d. 1914Königsberg) was an important physiologist. The lattices are
oftenfalselycalled ‘Hering lattices’ after the manwhomadeHermann’s discoveryfamous.
** See Hermann von Helmholtz, Handbuch der physiologischen Optik, 1867. This famous classic is
availableinEnglishasHandbookofPhysiologicalOptics,Dover,1962.Physician,physicistandsciencepolitician,
born as Hermann Helmholtz (b. 1821 Potsdam, d. 1894Charlottenburg), was famous for his works
on optics, acoustics, electrodynamics, thermodynamics, epistemology and geometry. He founded several
physicsinstitutionsacrossGermany.Hewasoneofthefirsttopropagatetheideaofconservationofenergy.
His other important book, Die Lehre von den Tonempfindungen, published in 1863, describes the basis of
acousticsand,like theHandbook,isstill worth reading.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net