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The second method of generating three-dimensional images is called Side-By-Side mode. In this mode, two images of the cave are drawn next to each other with no part of the image overlapping. Each image is then viewed with a different eye to produce the 3D effect. The side-by-side mode is useful because it can be printed and viewed with an inexpensive viewer. Since it does not require coloring printing, it is especially suited for publication.
Activating Side-by-side Mode. The Viewer normally defaults to Two Color Mode, so to select Side-By-Side mode, you must select the "Preferences|Set Stereo Mode" option from the menu bar. When you first select Side-By-Side mode, the second image of the caves will probably be off screen to the right. To view both images, you must zoom out so that both images can be seen.
Printing. Although some people can view this type of image on the screen, it is best to print it and then view it with a stereo-viewer. Since it is useful to be able to manipulate each image independently, you should either print the images on separate sheets of paper or cut the images apart.
Print Size. Side-By-Side images work best when the images are relatively small. This is because each image has to be placed directly under each eye. Since, your eyes are only a few inches apart, the images have to be placed just a few inches apart. This does not leave much room for large images. You can sometimes fold the paper to get particular parts of image under each eye. You can also view larger images using special stereoscopes that use mirrors to separate the larger images. (These are available from geological supply houses.) Finally, you can cut larger images into narrow strips that can be selectively positioned under each eye.
The Stereoscope. Once you have created the stereo pair, you will want to view it. Some people can view stereo pairs with the naked eye, but most people need a stereoscope for easy viewing. A stereoscope is an optical instrument that is used to view aerial photographs. It is also very useful for viewing stereo cave plots. The stereoscope is similar to the View Master viewers sold in toy stores. The instrument is very simple; it consists of two magnifying lenses mounted in a frame so that each eye looks through one lens. The frame usually has four legs, so that it can be positioned at a fixed distance above a stereo pair placed on a table. You can buy inexpensive stereoscopes at stores that cater to geologists and cartographers.
Using A Stereoscope. Learning to use a stereoscope is a bit tricky, and most people require some practice before they can use it effectively. To see the 3-D effect, you have to look at one of the images with your left eye and the other with your right eye. The tricky part is that your eyes automatically focus on one point, not two. As a result, you have to retrain your eyes to see the 3-D effect.
When you look at an object that is close to your face, say the end of your nose, the line of sight of each eye converges at a sharp angle on that point. If you look at a distant point, the line of sight of each eye is nearly parallel. When you look at a stereo pair, your brain sees it as a nearby object and focuses your eyes on a single point, when what you really want is to have your eyes looking at two different points, one on each photograph. The trick is to pretend that you are looking at a far away object when you look at the stereo pair. This makes the line of sight of each eye more parallel and allows each eye to focus on a different image.
Step-by-step Procedure. The following is a step-by-step procedure for examining stereo pairs:
1. Aligning. Before you can view the stereo effect, you must align the stereo pair properly. This means that the images must be placed so they are side-by-side in the same way they were plot on the screen. In other words, you cannot place the images top-to-bottom. If you align the images top-to-bottom, no stereo effect will be seen.
2. Positioning. The next step is to position each image directly under each lens. The easiest way to do this is to pick an easily identifiable landmark in the plot. It can be a passage, a station or anything that is easy to see. Locate the landmark on both plots and adjust the spacing between the plots until the two landmarks are about as far apart as your eyes. You may need to overlap the plots to get the landmarks in the proper position. Now place the stereo viewer over the plot so that the center of each lens is directly over each landmark.
Now look at the landmark through the stereo viewer. Close one eye and look at the landmark through the other. It should appear directly under that eye. If it doesn't, move the plot until it is in the correct position. Now do the same thing with the other eye.
3. Converging. The next step is to converge the two stereo plots into a single three-dimensional image in your brain. When everything is correctly aligned, open both eyes, relax and pretend you are looking at a distant object. As you do this, you should see one image drift toward the other. The further into the distance you look, the closer the images will get. The goal is to put one image right on top of the other. You may need to move the plot slightly, either up and down or closer or farther apart. When everything is just right, something magic will happen! The images will seem to lock together and the whole scene will jump into three dimensions. After you get the scene in stereo, you can move the plots or the viewer to other parts of the picture to examine the cave. If part of the image is hidden under the other picture, curl the edge of the top plot up so that you can view that part of the bottom plot. If you lose the stereo effect, move the viewer back to the landmark and realign everything.
Practice. At first, you may have difficulty getting and maintaining the stereo effect. However, with practice, you will train your brain and eye muscles to maintain the stereo effect easily, even when you move the photographs around. See the discussion on Stereo Viewing Tips for more help learning to see the effect.
Viewing Without a Stereoscope. If you don't have a stereoscope, you may be able to get the effect with the naked eye. Just follow the instructions above and skip the part about the stereoscope. It is easier for a nearsighted person to view with the naked eye because they can get close to the plot and still focus on the image. If you are nearsighted, take off your glasses before viewing the plots. If you are not nearsighted, reading glasses or a pair of magnifying lenses can help. The main thing is that you have to get your eyes fairly close to the page.