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When I was in high school (30 years ago), I took a biology class, and the instructor showed us an Ishihara color test for color-blindness. (This is the "hidden numbers" test.)

What I thought I saw once or twice was two different numbers* that had been embedded in the same slide. Were there slides in the test that had two or more numbers, or was I hallucinating?

[EDIT:] * "Two different numbers" means "different numbers in different colors."

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  • $\begingroup$ I got a chance to look through the actual images, and it appears I hallucinated the third number. $\endgroup$ – Christopher Heckman Jun 1 '19 at 22:15
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The Ishihara Color Test consists of plates containing one number. But that number may consist of two digits (Fig. 1). After scouring the internet I have also found variants of the test where the each of the two digit had a different coloring (Fig. 2), but these tests may not all be validated/standardized or used by health care professionals. The plates may also contain one or two wiggly lines that have to be traced (Fig.1). In all, my conclusion is that there are many variants of this specific test around, at least on the web. I am not aware whether there are certain standardized versions being used by health care professionals. In all, my answer to your question...

Were there slides in the test that had two or more numbers, or was I hallucinating?

...is that there might have been two digits of different coloring in the plate, dependent on the specific make of your teacher's plates. You might, or might not have been hallucinating. To give a more affirmative answer, the specific test with the specific make would have been dug up by your teacher.

number![wiggly
Fig. 1. Ishihara test with a plate containing one number of two digits (left) and one or more wiggly lines (right). source: Colour Blindness

Ishihara
Fig. 2. Ishihara variants, showing two digits with different shades in the middle panel. source: allinesnowmop, a personal web page, credibility questionable

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  • $\begingroup$ @user237650: Once again, I consider that one number. Maybe I need to add: "the second digit wasn't lined up with the first one"? $\endgroup$ – Christopher Heckman Mar 17 '19 at 6:36

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