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The division value of the convex mirror on the ruler is 0.5 mm,can easily slide, zoom in to read data, easier to accurately read small degree value.
For some old people, myopic eyes patients, it is very inconvenience when reading with a ruler. In particular,tiny dividing value degree is not clear, one convex mirror on the ruler can easily slide, so it can magnify the data to be read, reduce the probability of misread.And the division value of ruler is 1 mm,however,the division value of the convex mirror is 0.5 mm,can make it more accurate.