I will on occasion post some three dimensional images on my web pages to illustrate a point or make a physical item more easily understood. I have designed the images to be viewed in stereo with the naked eye. The old-time cartographers used to use this trick to view three dimensional aerial photos. It takes a little practice, but once you "get it", it becomes second nature.
This page describes how to view images that are arranged for parallel eye viewing. For images that use the cross-eye technique, follow the directions on my complementary web page, viewx3d.shtml. I also use the anaglyph technique to display 3D images. To view the anaglyph images you need to have special glasses with colored lenses as shown on my other page, viewanaglyph3d.shtml.
For the crossed-eye and parallel-eye images you can tell which type of image you are looking at by noting that above each image I have placed a small icon. The icons provide a focal point for attempting to merge the images and also tell you which technique to use when viewing the image. If you see icons which consist of circles with two parallel lines in them:
then you should use this parallel technique to view the image. If you see icons which consist of circles with two crossed lines in them:
then you should use the crossed-eye technique.
I have posted an image below that will allow you to practice. The marks above the images are there as an aid to learning the technique. The two circles with the ||'s in them will fuse into a single image with the || floating above the circle when you have properly relaxed your eyes. If the circle appears to be floating above the ||, then you have merged the images using the crossed-eye technique and the illusion of three dimensions in the image below will not work.
The image on the left hand side of the photo is the image that your left eye would see in real life. The image on the right hand side of the photo is the image that would appear to your right eye were you there in person viewing this scene. The trick, then, is to fool your eyes into seeing the merged stereo image instead of the two side-by-side images that you see now.
The reason that you see two images side-by-side instead of one merged stereo image is that your eyes converge on the image when you focus on it. This is a natural and unconscious act to help you see. You need to train your eyes to focus on the image, but remain looking straight ahead so that the left eye just sees the left image and the right eye sees just the right image.
Scroll the image to a place on the screen where it is comfortable to view. Now move your head toward the screen until your nose is touching the glass. Go ahead and do it, even if you look stupid. Looking stupid for a moment won't kill you. Put your nose right on bottom of the line that divides the two images. Once you have your nose on the glass, do not attempt to focus your eyes, just relax and let everything remain blurry. Got that? Try it again. Just totally relax your eyes and stare straight ahead through the screen to some distant place that you cannot see.
Your eyes should feel totally relaxed. If you are looking cross-eyed at the screen trying to focus on it, then you are doing it incorrectly. That will not work, and it will eventually give you a headache. Just let it all hang loose and go out of focus. Got it? Make sure that your nose is touching the dividing line between the left and right hand images. If your nose is on the glass between the two images, and your eyes are relaxed, you should see a single, blurry image in the middle of your field of vision. Try relaxing until you see the single image. Don't try to focus.
OK. The next step is to slowly move back from the screen. Do not attempt to focus your eyes. This is the difficult part because you are training yourself to look at something without focusing or really "looking" at it. Let the image remain blurry by just letting your eyes relax totally. Slowly move back from the screen while leaving the image out of focus and still looking far off, through the screen into the distance.
When you get about 3 or 4 inches away from the screen, the two blurry images should merge together into a single blurry image. Do not attempt to focus your eyes. If the images stay separate, and your eyes begin to hurt, that means that you are focusing on the image. Don't do that. Keep staring through the image and into the distance. Try this until you can do it. It may take several (many) tries.
When you are finally successful, the two blurry images will merge together into a single blurry, but definitely three dimensional image. Once you can see the blurry, 3-D image, congratulations, you are half way there. Remember what your eyes feel like right now. When you get so that you are viewing these images from a normal viewing distance, (and in focus), you will have to re-establish that feeling in your eyes.
The next part is perhaps more difficult than the preceding part because now you have to train yourself to focus your eyes while still staring off into the distance with your eyes parallel instead of converging on the image.
Slowly back farther away from the screen while keeping your eyes relaxed to keep the single, 3-D image merged. If the image suddenly snaps into the pair of images again, you have lost it and must start over up at the screen. Establish the blurry 3-D image, and then slowly back away from the screen keeping your eyes relaxed and the image merged. When you get about 12 to 14 inches away from the screen, the merged 3-D image will slowly come into focus with no effort on your part. If you try to focus your eyes on the image, you will lose the 3-D. Just stay relaxed, and it will happen.
When you "get it", there will three images in your field of view, a flat image on the left and a flat image on the right, and the 3-D image between them. Just ignore the two extra images and enjoy the 3-D image.
Once you have managed to see the 3-D image in focus, pay attention to what your eyes feel like. They should be totally relaxed as though you are staring off into the distance in a trance. Eventually you will be able to just relax your eyes and see the image in focus and in stereo without having to move back and forth. All you have to do is re-establish that relaxed feeling in your eyes. I find it to be quite relaxing to do it, and I will actually let my eyes go like that on occasion just to give them a break. It feels good. If it hurts, you are doing it wrong.
If you have tried and still cannot do this, try practicing on some of the Magic Eye images here http://www.magiceye.com/enter.html. (I have no financial relationship with Magic Eye, and I provide this link only for your convenience in trying to learn this skill).
Some people are never able to do this trick. The instinct to focus on an image is just too strong and it won't work. If that is the case for you, don't worry. Just enjoy the images as they are.... colorful but flat.