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With a small purple dot in the middle of a sheet of white paper. When you hold it close to your eye, you should see a blue dot with a red fringe, and as you move it away, the dot should appear red surrounded by a blue halo.
Hold a wire about 1mm very close to one eye with the other closed.
With your eye relaxed - look at the tip of your finger so it crosses the wire - it will not connect except at one distance - this is the normal focus of your eye. You can change the angle of the wire and find a max and minimum distance - the angles (at right angles to each other) are the axis of astigmatism.
This is the basis of Thomas Young's optometer - They say he used a double slit - but his double slit experiment used a 'card edge' of 1/30th of an inch (.85mm) - could be his optometer used the same?
looking though a pin hole will help you see floaters. Bits of stuff in your eye..
Looking via pin-hole and squint to see circle of pupil block unblock change iris size..seeing shadow of iris.
A phosphene is the perception of light without light actually entering the eye, for instance caused by pressure applied to the closed eyes.
Attributed to the dichroism of the xanthophyll pigment of the macula.
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