In this illusion, the line at the top appears to be longer than the one at the bottom of the image, even though they are the same length. Why? Because even though there is no depth in this image, the converging lines activate the brain’s depth perception system, making you interpret the two yellow lines as receding into the distance, like the rails of railroad tracks.*
How might this impact on your perception of relative object sizes affect your ability to select a vial of the correct strength?
* Palmer SE. Vision Science: Photons to Phenomenology. Cambridge, Mass.: The MIT Press; 1999.