NARRATOR: The name quot;Perkinsquot;carved in stone. Below a gothic tower,a boy navigates with a cane. LUISA MAYER:We do differentiate between visual acuity and visual field, visual acuity being thecentermost part of vision where we focus on things,we fixate, we see details. Everything else isthe visual field. Even though it's really onecontinuous surface,
there really is no separationin our vision or in our minds between visual acuityand visual field. Some important things about thevisual field is first of all, as I showed you, if you do this,you hold your finger when you can't see it,you bring it forward. And you see it, you're awareof it out in your field, and out here. That's the fullest extentof your field.
And it's actually a littlemore than 180. It's actually a little more than90 degrees from fixation, which is where we really liketo count in this field. NARRATOR: We see a pencil sketchof an adult male face headon, with the left eye coveredby the left hand. A shaded, semicircularwedge shape depicting the horizontal rangeof the right visual field extends from the eyealong a horizontal plane
and is bisected by a line,dividing the wedge into the nasal and temporalfields. The nasal field is describedby an arc of 60 degrees. The temporal field on the otherside of the axis is 100 degrees for a total range of 160 degreesin the right eye. MAYER: And then when you'rethinking about the upper field, the farthest position up you seeis about 60 to 70 degrees. And the farthest positionyou could see out,
the farthest amount of degrees,is actually way down here, but I'll bring it upso my finger is seen. It's 80 to 90 degrees,so it's very big. NARRATOR: A second sketch showsan adult male head in profile. A shaded, semicircularwedge shape extends from the eye in a vertical plane,and is bisected by a line representing thehorizontal axis, dividing the wedge intoupper and lower fields.
The upper field limitis 60 degrees from the axis, the lower field 75 degrees, for a total rangeof 135 degrees. MAYER: Your vision off to theside is coarser or rougher, and you don't see colorsas well, you don't see details really very well at all. You do see big, gross patterns,you see contrast differences. So, for example, I can seethe wall contrast over there
How Do Chameleons Change Color
There is a misconception about chameleonsthat they change their color in order to blend in with their environment. That is actuallynot the case. When a chameleon is calm it is green and so it naturally blends in withits leafy surroundings. But male chameleons change color when they become excited in thepresence of a female or a rival male, as shown here. I will put the first frame in the bottomright for comparison. As you can see, as he becomes more excited, his color changes tobe more yellow orange and red. So instead of blending in this chameleon will now standout. But how do chameleons actually change their coloré For years we thought that therewas a simple wellestablished mechanism for
this color change. In their skin, chameleonshave different cells containing colored pigments. Some are yellow others are red and some containthe dark melanin pigment. Many animals including chameleons are known to turn a darker shadeby causing the melanin to spread out along the fingery extensions of the cell. They turnlighter again by condensing the pigment back into one spot. It was assumed that chameleonsmust turn yellow, orange and red in the same way, by causing those pigments to spread outin those cells. But new research from collaborating groups of physicist and biologists at theUniversity of Geneva shows that this is incorrect. Now the first clue should come from the factthat there is actually no green pigment in
the a chameleon. The green color actuallycreated by two distinct mechanisms, pigment color and structural color. Beneath the yellowpigment cells, there are cells containing tiny 130 nm crystals regularly arranged ina lattice. Now light diffracts off these crystals and due to the spacing between them, bluelight constructively interferes and is therefore strongly reflected. Whereas the other colorsare not reflected. This is very similar to how a morpho butterfly wing creates an iridescentblue. There is no blue pigment just the periodic nanoscale structure that acts as a strongreflector for only one color. So a chameleon looks green due to the combination of theyellow light from the pigment plus the blue
light reflected off the crystal structurebeneath it. So how do chameleons change this green into yellow orange and redé Well previouslyit was assumed that the color change was achieved by dispersing pigments in the colored cells.But now scientists have found it's actually the crystals underneath which are changing.By increasing the spacing between the crystals the chameleons can change which color is selectivelyreflected. Bigger gaps between the crystals are better at reflecting longer wavelengths.So as the spacing increases, the color changes from blue to green to yellow, and then orangeand red. Scientists compared skin samples from calm green chameleons and excited yellowchameleons and found that indeed, in the excited
chameleon the crystal spacing was much furtherapart. Here you can see a single cell enlarged and the color it reflects changing from blueto green to yellow, orange and red. Plus, a computer simulation of the light reflectedoff crystals while their spacing is decreasing also provides excellent agreement with theseobservations. Furthermore, applying physical pressure to the skin causes the crystals tocompress and so they reflect more blue. When that pressure is removed, the crystals expandagain reflecting more green light. The structure of a chameleon's skin gets even more fascinating,as there is another layer of cells underneath which also contain crystals. But these crystalsare larger more spaced out and more disorganized.
This means they reflect longer wavelengthsin the infrared part of the spectrum and they reflect a broader range of these wavelengths.It's thought that the function of this layer could be to reflect light from the sun sincethese chameleons live in bright fullsun habitats. So male chameleons don't change their colorby spreading out red and yellow pigments. Instead they do something much more remarkable.They actively tune the spacing between nanoscale crystals in order to create structural colorsthat span the whole length of the visible spectrum. Now what I find amazing is thatin this day and age we thought we had the answer to how chameleons change color, butit actually took the combined expertise of