Would you like to see drawings like this in my astronomy pages in three dimensions? If yes, please follow these simple instructions.
Select a spot at the opposite wall: it could be the door knob -- or anything at a distance of more than 2 m. (7 ft.). Place your two index fingers at arm's length as shown in the figure above, while watching the door knob between your two fingers.
If you are focusing on the door knob, then you actually see FOUR fingers!
If you don't believe me, shake your fingers slightly, so their four moving images will become apparent. Don't focus on them -- the trick is to be aware of all four of them while still focusing on the door knob. If you feel your eyes become de-focused from the door knob and focus on your fingers whenever you move them, have patience! The experiment will not work if you cannot pass this first step.
Are you in a position to "see" four index fingers while focusing at a distance? (There is nothing wrong with your eyes: this is the way our visual system works to perceive depth, in the 3rd dimension.) Now move your fingers inwards slowly (see figure below), until the two innermost ones "merge" into one.
Remember, keep focusing at the door knob: I admit this is a bit difficult to do since your fingers get in the way, but nothing prevents you from repeating this movement as many times as needed. Practice doing this. If you have successfully "merged" your two innermost fingers you are effectively seeing THREE fingers now (shake them a bit to see them better). We will need to refer to this position, so let us call it "Position A".
Now the following action is the key to our experiment.
Since you still focus at a distance, slowly de-focus (without moving your hands) and look (focus) straight on one of the two fingers (or in the space between). Do the de-focusing slowly, feeling how your eye muscles move while you change focus from door to fingers. This is important, I am not joking. It is important because now I will ask you to do the reverse movement! Try to bring your two innermost fingers back to Position A (as they were before you de-focused) without moving your fingers, just your eyes. Can't do it? Don't be discouraged. And read on, you are very close to completing the job.
I assume you had some difficulty in doing the reverse movement (merging your two innermost fingers without moving them). So here is what to do.
Go back to Position A, and this time do the de-focusing in a very careful and controlled way: Do not de-focus all the way, but only a little, hesitating, and going back to Position A immediately. Do this several times, seeing your fingers becoming separated and merged again. As you become more confident, gradually increase the separation distance while you move your fingers apart-and-together (remember, no other part of your body moves, only your eyes). As you become more and more familiar with this movement you will find yourself being able to separate them all the way, and bring them back to the merged position (Position A) at will.
You have essentially learned to control your eye muscles. Don't worry, there is no problem of getting in some kind of eye-strain trouble because of muscle contortions: you don't contort anything -- this is the natural position of your eyes when you focus at a distance (except that usually there are no other objects -- such as fingers -- in front, so you don't have any chance to notice it).
Moving your eyes at will so that they come to Position A is all this text is attempting to teach you. Why is this sufficient? Because if you can merge your two index fingers at will, you can also merge the two almost-identical-but-not-quite parts of a three dimensional figure, such as the two parts below.
Just pretend they are your index fingers, and do the same thing. You will know when you manage to merge them, because you will see the yellow ball standing out in front of the page, while the red one stands in the middle, and the blue one stays in the background! You may find it easier to do the merging if you position yourself at some distance from the screen (e.g. at 1/2 meter, or 1,1/2 feet). Isn't this much better than red-and-blue glasses? (which eliminate the colors, by the way).
The 3D images that you saw in the astronomy pages are more demanding, since the separation is greater, but with enough practice you will soon be able to see them properly.