to collect in a mass

When someone says they have, for example, 20/40 vision (this is US-specific, apparently), do you know what those numbers mean?  I didn’t until just now.  I just renewed my driver’s license online for the first time (no waiting in line at the DMV: nice!), and they asked me to certify that my vision is no worse than 20/40.  There was a footnote that said you have 20/40 eyesight if you need an object to be 20 feet away to be as clear as that object would be if it were 40 feet away for someone with normal vision.  The standard chart you’ve probably had to read at one point in your life (pictured above) is called a Snellen Chart:

This line, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet.

Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of twenty feet, but a person with normal acuity could be expected to read these letters at a distance of forty feet. This line is designated by the ratio 20/40. If this is the smallest line a person can read, the person’s acuity is “20/40,” meaning, in a very rough kind of way, that this person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet. In an even rougher way, this person could be said to have “half” the normal acuity.