Bertrand lens

[Plasmid]

Can someone explain ( in the easiest way possible) what are Bertrand lens and their application!

[PeteM]

A Bertrand lens is usually built in to a microscope - and focused so you can see (and adjust) the alignment of things like phase rings and phase annulli in a phase contrast microscope.

A centering or phase telescope - which replaces an eyepiece - does the same thing. Because the focus point is adjustable (the "telescope" part), you can also peer down into and inspect various surfaces in the optical train for things like defects and dust - as well as showing the alignment of a phase microscope, the alignment of irises etc.

Best way to learn about them -- if you have a need -- is to find a good used one on Ebay for $50 or so and try it out.

If you have a perfectly functioning microscope (not phase) already and a limited budget, you could save the $50 for something else.

[Plasmid]

Now that I can understand, thank you so much.

[Greg Howald]

A Bertrand lens is akin to a petrographic microscope used in mineral identification. It actually reduces the view to a smaller area in the center of the field of view. A pattern of a cross is seen as the lens divides the view into quadrants. With mica the cross is perfectly centered. The cross is not centered when using other minerals. Then you usually see only one quadrant at a time as you rotate the stage 360 degrees. By adding the quartz filter you record what is seen in each quadrant to identify the mineral. There's much more to it than that, 1/4 Phase filters as full gama filters, but that is about as basic as I can be.