Am I the only one who uses the cross-eyed method of comparing lists? I mentioned this to someone at work today and they looked at me like I was a space alien.
The way it works is this: you take the two lists you want to compare, and line them up side-by-side. You can do this with sheets of paper, or on-screen with columns of data in spreadsheets.
Next, slowly cross your eyes so that the two lists are superimposed. The hardest part of this is crossing your eyes and keeping them focussed at the same time. But if you can get the two lists exactly overlapping in your cross-eyed view, it becomes very easy to spot any differences. Items that are identical will overlap precisely, and will look normal. But items that are different will never properly line up, and so will look strange.
Here are two lists you can try this with. You can print out, or you can go cross-eyed at your computer screen. There are ten items in each list, and two differences between them. How quickly can you spot them?
| Elephant | Elephant |
| Antelope | Antelope |
| Alligator | Alligator |
| Kangaroo | Kangaroo |
| Hippopotamus | Hipopotamus |
| Cassowary | Cassowary |
| Okapi | Okapi |
| Meeerkat | Meerkat |
| Platypus | Platypus |
| Flamingo | Flamingo |
The effect is a bit like those “Magic Eye” images that were popular a few years ago. The overlapping lists seem to have an odd 3D quality to them.
This comes about because of the way we perceive depth. Our eyes are set about 10cm apart in our heads, and so each eye gets a slightly different picture of the world. Our brains put the two images together into what we know as 3D vision. This is also how stereo photography, or stereoscopy works. Using a special camera with two lenses, set slightly apart, you take two pictures of a scene. Then, using a stereoscope you place the left image in front of your left eye, and the right image in front of your right eye. Your brain then combines and interprets the two inputs, and makes you believe you’re looking at the original scene, in 3D.
But if you don’t have a stereoscope, an alternative way of seeing the 3D effect is to look at the two images side-by-side, then cross your eyes until the pictures superimpose. Doing this doesn’t reproduce the original 3D scene perfectly because your right eye is looking at the left image, and vice versa, but it does give the illusion of depth.
Doing the eye-crossing thing on two lists, or other images that don’t represent a 3D scene (when I was younger I used to use this method for doing “spot the 10 differences” type puzzles), don’t produce a 3D composite scene in your brain. But there are usually subtle differences in the texture of the paper you’re looking at that give it a slight 3D “effect,” so that the words or picture you’re looking at will appear to shimmer slightly above the background surface.
It makes for a nicely efficient way of comparing lists, provided you are able to focus your eyes at the same time as crossing them…
Since I can barely get a “Magic Eye” trick to work after fifteen minutes, crossing eyes to compare lists is not time-efficient for me.
What I do is to print the lists out, one on a page, in the same font and size, then lay the pages atop one another and look through them at a window or light. Once you have the first element precisely lined up, any differences leap out.
(This technique is useful, as well, for determining if two pages of text are photocopies of the same sheet, based on the stray marks copiers can leave. I needed to do that from time to time when I was a paralegal.)