NERVOUS SYSTEM AND SENSES
Cut a piece of white cardboard with the above dimensions. With a black pen, draw a black cross and a black dot as shown above. Close your left eye and hold the cardboard 30cm away from your face. Stare at the cross. Move the cardboard very slowly towards your face. At some point the black dot should disappear.
At the back of the retina of the eye is the "blind spot". This is the place where the optic nerve joins the retina. At this point there are no light sensitive nerve cells, so you are "blind" in that spot.
Stare at a red object for at least two minutes. Then look at a white sheet of paper. What do you see?
You should see a red-coloured image. This is because there are four types of light-sensitive cells on the retina at the back of your eye. There is one type of rod-shaped cells that responds to shades of greys, and there are three types of cone-shaped cells, one responds to green, another to red, and yet another to blue. The cones that respond to red light tire quickly so they stop working for a while. When you stare at the white paper only the green and blue cones are working, so you see a greenish blue colour called cyan.
Try it again by staring at a blue object, only the red and green cones will be working so you see an after-image of yellow.
Contracting and Dilating Pupils
Go into a room with a mirror and draw the curtains or blinds. Remain in the dark for a few minutes. Then quickly switch on the light while you are looking into the mirror. What happens to your pupils (the black dots of your eyes)?
Your pupils are holes that allow light to pass into your eyes. In the dark, your pupils dilate or enlarge to allow as much as possible to enter the eye so that you can see. As you switch on the light, your pupils contract or get smaller to allow less light to enter.
It is also thought that your pupils enlarge as you read an interesting book or look at an interesting picture. Test this on a friend but don't distract them from what he/she is doing.
Do Bees See Colours
Cut squares (about 2cm X 2cm) out of the coloured cardboard sheets. Glue this randomly onto each of the grey cardboard sheets. Allow to dry. Then mix up some sugary water and pour into each of the 4 trays. Into each tray, place one floating plastic plate and put the cardboard on top. Set these up outside in a place where you can observe through a closed window. Wait for a bee and observe if the bee lands on one colour more than any other.
Bees will probably be more attracted to the blue colours.
Find the Nerve Receptors in your Skin
Gently stick a sewing pin into the skin of the palms of your hand as close together as you can. Now try it on your arm. What do you notice? Throw it away in a secure place after you have finished with it so that diseases cannot be spread.
You will probably find that the skin on the palms of your hands is very sensitive compared with the back of your arm. This is because there are more nerve receptors on the palms of your hands than on the back of your arm.
Sight versus Sound Reflexes
First using just sight Hold a ruler by the top at arm's length. Get a friend to place his/her fingers at the bottom of the ruler ready to catch it when you drop it. Don't warn him/her when you drop it. Measure the distance the ruler has dropped before your friend catches it.
Second using just sound...Blindfold your friend. Repeat the experiment but this time say 'Go' as you drop it. Was your friend faster or slower than with just sight?
Apple, Potato and Onion Tasting
Chop each of an apple, a potato and an onion into the same sized pieces. Do not let your friend see what you are doing. Refrigerate them for a short while. Now blindfold your friend. Tell him/her to pinch his/her nostrils tightly so he/she can't smell. Feed them the different pieces of fruit or vegetable and ask your friend to guess what each one is.
An important part of our taste is to smell our food. Your friend will probably not be able to pick the apple from the potato because they are the same texture. However, he/she will probably pick the onion but because of its firmer texture not from the taste.
Put a blindfold on a friend. Set up 3 bowls of water - the left one of cold water with ice cubes, the middle one at room temperature, and the right one should be hot but not hot enough to burn. Put your blindfolded friend's left hand in the cold water, and the right hand in the hot water for a few minutes. Then remove both hands at the same time and place them into the room temperature water. Ask your friend what he/she feels in each hand.
The hand in cold water should suddenly feel hotter than it should in the room temperature water, and the hot hand should feel particularly cold.