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# Astronomical question and answer 273

Frank L. Preuss

What mistake can be made, when the distance between the two eyes is measured?

The line between the two eyes must be parallel to the ruler.

The distance between the two eyes can also be measured with a ruler fixed to the mirror, when, looking into the mirror, the head is for example turned to the left. Then the right eye is closer to the mirror than the left and the distance between the two eyes that is then measured is less than the real distance.

And that is, because the eyes in the head can turn and the line between the eye in the head and the eye seen in the mirror is still perpendicular to the mirror. And the two lines, therefore of the left eye and of the right eye, are also parallel, but the distance is less than the actual distance.

This is the end of "Astronomical question and answer 273"
To the German version of this chapter: Astronomische Frage und Antwort 273

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