Believe the instrument

[newt]

Today I was cutting some dovetails in pine. The pine had very prominent growth rings on the end grain, which were symmetrical about the centre of the board. When I had marked off the tails across the end grain something did not look right. When looking at the end grain of the board the lines were seen to not be at a right angle which appeared worse at each end of the board. The line nearest the centre looked ok and this happened to be in the middle of the growth rings, like a line hanging from the centre of a church door. As you moved towards each end of the board the angle of the rings increased.

I checked the board again everything was square but the illusion persisted. I asked my wife to look and without prompting she experienced the same effect. When I learnt to fly I was always told to believe the instruments not what you think you can see or feel. Anyway I cut the tail board but it still looked wrong, so I stuck some white paper across the end of the board and then sliced the paper through into each cut in the board, and guess what every thing looked spot on.

In conclusion the effect was caused by the changing angle of the growth as the eye moved to each end of the board. I have not noticed this effect before but there again I have never cut dovetails in pine, has anyone else experienced this effect.

 

[engineer one]

well done newt, you have just experienced why so many people put
up mirrors and paintings on the slant. :?

it is always easier to try and obscure the patterns to ensure that your
cuts are straight and correct.

paul :wink:

 

[Paul Chapman]

Hi Newt,

Yes, I've found this before. Often happens when you draw a straight line on a piece of wood with very wavy grain. It just doesn't look right, no matter how many times you check it. I like your idea of covering the wood with plain white paper :wink:

When I learnt to fly I was always told to believe the instruments not what you think you can see or feel.

I know what you mean. My son is training at the moment to be a commercial pilot (which explains why I can't afford many tools ) and is going through the stage where they blank out the windows and you just use instruments - quite scary :shock: :shock:

Paul

 

[newt]

Thanks for the reply,I am glad you understood my explanation, a picture would have been better. There is a saying, if it looks right it probably is right, but clearly not in this case, the eye can really deceive.

 

[engineer one]

ah but newt that really means a complete object, whilst parts can
be off completely yet make a fantastic object.

there is a famous story of george brough of the rolls royce of motorcyles
fame checking machines the night before an important exhibition.

he said to the fitter "the petrol tank is wrong" fitter says "well it's too late
to fix now" so george takes a hammer to it, and says "there is now!" :twisted:

if you feel your eye is being deceived then try to remove the extraneous
problems and you will be better able to see it. so your white paper trick
looks really sane.

paul :wink:

 

[newt]

Paul,very true the saying does refer to the whole rather than a component part.

 

[Jorden]

Hi Newt,
Yes I've been there both descending through cloud and working to marked lines over growth rings. The scariest one has to be being convinced the airplane is turning hard left when the artificial horizon says you're level and descending though

 

[garywayne]

The eye is never wrong, it's how the brain interpritates/ interprets, reads it.

 

[engineer one]

whilst i would like to agree gary
maybe you should ask an optician whether the eye is always right :lol: :lol: :twisted:

paul :wink:

 

[garywayne]

Ok Paul I agree it can be a bit fuzzzzeeee at times.

 

[engineer one]

deuce i think, btw how's your leg??

paul :wink:

 

[garywayne]

Thanks for asking Paul. The insurance bone specialist person has just reported that I will probably be like this for the rest of my life. I am not convinced. I don't mind the hobbling, It's the pain I don't like.

A bit off track. SORRY for rambling.

 

[newt]

I think the eye does not see, its the brain that computes the image, the eye is just a sensor. Mind you sensors can get corrupted.

 

[OllyK]

well done newt, you have just experienced why so many people put
up mirrors and paintings on the slant. :?

Don't get me started on that! Our house is all out of level and square, quite significantly for the most part. I put pictures up, using the spirit level and the first the SWMBO says is "that's not level". My response is "it is level, it's the house that isn't" but she won't have it, so we have curtain poles and pictures on the pi$$ all round the house and it drives me nuts!

 

[engineer one]

but olly think how p*****d you would be if the house was brand new
and the pitcures weren't straight, won't mention any
major housebuilders but we all know who i am talking about :lol: :lol:

paul :wink:

 

[newt]

jordan, it really is weird when you are convinced you are in a turning decent but the instruments tell you are level. I was involved in some military research and many pilots have been killed when in a panic situation they reverted to "seat of the pants mode" rather than instruments. Safe flying.

 

[Jorden]

Hi Newt

Apparently if I were flying an aircraft which had a propeller turning in the other direction the effect would have been that of turning and diving to the right. The airplane was properly trimmed so the only thing I can think of that would be different is the swirl of water vapour passing over the canopy. Still it just goes to show how easily the eye can be deceived!

Dennis