[John Chapman (OP)]

Hi yor_on

Are you suggesting that the reason looking at a bright light creates a blotch is because it illuminates blobs of 'stuff' floating within the vitreous humour that aren't normally visible? Why would we still be able to see them after the eyes are closed? I think what you are describing is a different phenomenon to the one I mean. Are you talking about what I think are known as floaters? I don't know why you see light when you squeeze your eyeballs but I always guessed that this increases the pressure within which somehow triggers the rods. I've often also wondered why you see flashes of light when you bang your head or even stand up too quickly. I may post this as a separate question.


Hi Techmind

That's a really good answer. So that's you, me and Chem4me who have given ourselves migraines peering into our torches. Do you think we could all sue Chris Smith if we go blind by experimenting for his site!!! (only joking, Chris)

What you said about laying in the sun was interesting:

... if you've been laying in the sun for a few minutes with your eyes shut (you'll see the bright red of the blood in the eyelids), when you open your eyes and look around everything will look bluey-green (and completely lacking in red) for a few minutes until the red-receptors have recovered.

So that's sensory adaptation at work.

But what really interested me was your explanation for the colour change. I assumed the significance of having your eyes open or closed was the light or dark environment within the eye. But your experimentation suggests that it is caused by mixing the colour of the blotch with different background colours, the colour of 'eyes closed' being either red or black depending on ambient light conditions. That's excellent.

[Make it Lady]

I think your colour detecting cone cells are recovering from the exposure to bright white light at different rates. Because white light stimulates all your colour detecting cells they stop sending signals to your brain. As some of them recover your spots will take on colours.

You are not a freak.

Try this spooky experiment but don't tell Paul as he will transfere it to kitchen science.

Colour Change Casper
If your Halloween party is becoming rowdy try this eye tricking experiment. Children have to concentrate on a picture of a ghost. This usually quietens them down as concentration requires quiet!
 
What you need:
   Some sheets of blank white paper (A4 is fine)
   A thick red and a thick black felt tip
   A bright light (sunlight or a table lamp)

What to do:
1.   Draw the outline of a ghost (about 8cm high) on the left hand side of a blank piece of white paper.
2.   Colour in the ghost with the red felt tip.
3.   Hold the paper with the ghost on it at arms length.
4.   Make sure a bright light is shining on the paper either by standing close to a window or by pointing a lamp at the page.
5.   Stare hard at the ghost for about 30 seconds. Stare at the same place in the centre of the ghost. If you need to blink do it quickly.
6.   After the 30 seconds are up, blink your eyes quickly once and then stare at the right hand side of the paper. This area should be blank but you may see something strange.
TIP: You should see a green ghostly image but if you don’t try blinking a few times and staring at white paper again. If this still doesn’t work repeat the experiment again with a brighter light shining on the paper.

I exSPECTRE want to know what’s happening:
The reason why you see a different coloured image after staring hard at a coloured ghost is that you have tired out the light receptor cell at the back of your eyes. Coloured light detector cells are called cones and no you can’t eat ice-cream out of them! There are three types of cones. Each type detect either red, green or blue light allowing you to see all possible colours by colour mixing. White light is made by combining red, green and blue light so all cones are stimulated when you see white light. If you stare at a red ghost your red cones will get tired and the longer you look at red the less signals the red cones send on to your brain. When you change and stare at the white paper the green and blue cones tell the brain that they are being stimulated but the red cones are too tired to send any more messages for the ghost’s position on the eye. You should now see a blue/green ghost floating on white paper as only the blue and green cones are sending messages to the brain where the ghost was positioned. Repeat the experiment with a black ghost.

[yor_on]

Hmm, maybe you're right John:)

But I have a memory of them becoming more visible when looking at bright spots.

John I didn't say that you would see 'floaters' when pressing on your eyes.
I said that that colours and shapes you will see then is a way of your brain interpreting electrochemical stimuli coming from your optic nerve due to the pressure placed on those eyeballs by your fingers.
 
I will split my first comment in two so that one can see that I was talking about two things more easy.

I also have a vauge memory that if you close your eyes with a bright light shining on them you will also be able to see those 'floaters'

http://www.triadpublishing.com/eyecarereports/floaters.shtml

[John Chapman (OP)]

Thanks for that, yor_on. And that link was very interesting.

Sorry about the confusion with your previous post. Your comments were clearly and intelligently put. It was my response that was misleading.

Yes, I realised you weren't suggesting that floaters are made visible when your eyes are closed or by applying pressure. I was pointing out, though, that my blotches are. Which is why I said that I thought that my blotches are the result of a different phenomenon to yours. Because I referred to all your comments using a single paragraph I seemed like I was mixing them up.

[John Chapman (OP)]

Hi Make it Lady

That's fantastic! I thought we had this licked with Techmind's answer. But it really seems like you might have hit the nail on the head. And very entertaining too.

At the stage where you look at the blank paper and see the blue/green floating ghost, what happens if you close your eyes and cover them so they are in darkness. Do your eyes continue to generate an image? And if so what colour is it?

I'd like to hear your answer but I'll try it myself as well.

[Chemistry4me]

Mr Chem?
Does this mean that you see glades as a threat to our security?

Eh? I don't know what you mean [][]

-----
Thanks for the reply Mr.yor_on []

[Chemistry4me]

Hi Chem

That would worry me. Are you a fit person? Has this happened before?

I would be tempted to check that out with my GP. Be careful. []

It has happened once, at exactly the same event: the 100 m sprint! All the other running is fine but I must not have warmed up properly or I am simply just not used to the explosive nature of the 100m!

[Make it Lady]

I quite often get a rush of blood to my head after sprinting (not that I do it any more, far too creaky.) You must remember that you do have blood vessels feeding your eyes so sudden changes in blood pressure can change your vission.

 John I've not tried out what you said about shutting your eyes. I will let you have a go as I have been doing eggciting eggsperiments al day with 5 to 11 year olds. I'm bloody knackered.

[yor_on]

Mr Chem?
Does this mean that you see glades as a threat to our security?

Eh? I don't know what you mean [][]

Well Sir, there are some associating open places with danger, and as we already established the connection is there (exhaustion/danger)...
It all depends of course, in a serious 'situation' that feeling might be shared amongst all involved I would say?
It can't help but start me wondering again, though?

You wouldn't, by any chance, enjoy a nice swim and boating now and again?
Ah, like those fellows? http://www.specialboatservice.co.uk/
Sounds like they're giving a really good service, although I couldn't for the world find out about their fares?
(Awh, it just sort of 'popped up'

[Make it Lady]

Are you on drugs?

[Chemistry4me]

Are you on drugs?

F*** no!

[Chemistry4me]

You wouldn't, by any chance, enjoy a nice swim and boating now and again?
Ah, like those fellows? http://www.specialboatservice.co.uk/
Sounds like they're giving a really good service, although I couldn't for the world find out about their fares?

Well I do enjoy an odd boat and swim around the harbour.
The Special Boat Service certainly looks very attractive.
You know how I am with the Navy/Army. []

[chris]

Quick. Someone come and rescue me. Chem4me is making me think I'm a freak. Surely someone has experienced this colour change phenomenon. Anybody??

This is true - I too have experienced colour changes like this. It's down to retinal processing I think. The retina "tunes" itself to maintain the best visual signal and optimum signal to noise. If the photoreceptors are bleached by intense light (which paradoxically shuts them off rather than turns them on) then there is a compensatory re-tuning effect in the deeper layers of the retina, producing different colour perceptions.

Chris

[techmind]

I've sure I've posted it before, but here's a cool illusion that'll demonstrate some of the weird stuff the eye/brain does...
http://astarte.csustan.edu/~tom/cool-stuff/

[John Chapman (OP)]

Hi Chris

Thanks for your support


Techmind, that's interesting. That illusion involves generating a green colour in place of the purple. Just as happens with my blotches. That can't be a coincidence. Any idea what the mechanism is?

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[yor_on]

Are you on drugs?

Nope, on a couch.

[Chemistry4me]

[ Invalid Attachment ]

Hey I can see the green dots moving if I try hard enough.

[Chemistry4me]

Are you on drugs?

Nope, on a couch.

Is couch a secret code name for drugs? []
Btw, that's my drugs couch!

[yor_on]

Nope no drugs.

Just goodies:)

Have a beer on me then, Mr Chem.
(As you already  seems to have taken my couch:)

[Chemistry4me]

Just goodies:)

What on Earth? Who on Earth?