Why does the outline of your finger look like its been drawn with pencil?

[Rebecca Was here]

I've found out that when you look at your finger closley you can see a line thats like its been drawn with pencil.

 

[Kate]

You do realize that if you focus your eyesight on something hard enough, that the background falls away and darkens so it just LOOKS like it was a line? Stop focusing that hard, it's actually bad for your eyes.