Hi, I have a question about resolution as defined by rayleigh's equation r = 0.61 x lambda/numerical aperture The maximum half angle of acceptance of a lens is 90 degrees and the sine of this is 1. At this point the only thing that can increase the value of the denominator in the equation is to increase the refractive index of the imaging medium e.g. Immersion oil. So if you could find a medium with a higher refractive index than immersion oil (1.515) then according to the equation this would make the minimum resolvable distance between two points smaller. This would also mean tho that any other medium between the object and lens would need to have the same refractive index or at least close to minimise spherical aberration. So I'm guessing immersion oil is as good as we can do? But why is that? Many thanks.