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Combination Vision Test

Title text: If you see two numbers but they're both the same and you have to squint to read them, you have synesthesia, colorblindness, diplopia, and myopia.

[edit] Explanation

There are two numbers embedded in the big circle of numbers, in a similar way to a common color perception test. But this test can not work for colors because it is just a Black and White picture. Nobody can see it. But at this point the joke is just starting:

The numbers are four and two, forming the number 42, which is the famous “Answer to the Ultimate Question of Life, the Universe, and Everything”, according to the book The Hitchhiker's Guide to the Galaxy.

The number 4 is formed by digits 2, 3, 5 and 7, which are the only prime numbers in the range 0–9. The number 2 is formed by digits 3, 5, 7 and 9, apparently all odd numbers in the range 0–9, except for 1.

A normal person does not readily see the big numbers, obviously. However, there is a neurological condition called synesthesia, in which perception in one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. Common examples are experiencing colors when seeing numbers or words, hearing tones or music while reading words or text, seeing sequences of numbers or month names in a distinct and fixed shape, etc. If we give some numbers a color, such as how a person with number to color synesthesia would see it, we get this:

The comic playfully suggests that if you have synesthesia as well as colorblindness, then some of the colors might appear identical and so one number would not be visible, only leaving the other number.

The alt-text brings in two more conditions: diplopia, or double vision, and myopia, or near-sightedness. Those who are near-sighted sometimes see distant objects more clearly while squinting. Then they would be able to see the one large number still visible from the synesthesia/colorblindness combination, but because of double vision they see a second copy of it, hence two numbers that are the same.

Since each number would appear in a different color with synesthesia (unlike above where 2,3,5,7,9 are all the same color), the following image better demonstrates how the image might appear to some viewers.

[edit] Transcript

Combination Vision Test

a circle filled with several hundred numeric digits

If you can see one big number but not the other, you have synesthesia and colorblindness.