set xrange [0.25:1.0]
set yrange [0.8:300]
set logscale y
set title 'approximate perceptible aberrations for 160mm TL objectives'
set ylabel "allowable tube length error (mm)"
set xlabel 'numeric aperture'
set xtics 0.05
set grid xtics mytics
plot .89253125 / (x**4) t 'immersion'
replot 5.77200625 / (x**4) t 'dry'