Fresnel Biprism Equation Derviation- please help!!

[physicsstudent]

I recently did the Fresnel Biprism Experiment in order to calculate the wavelength of sodium light. As part of my write up I am to show the derviation of the formula d = (d1 * d2)^1/2 where d = actual separation of the virtual slits. Basically I dont have any idea how to derive this, all I know that it is supposed to come from the simple lens formula......does anyone have any idea how to derive this?? Any help would be greatly appreciated!!

[Integral]

I am looking at this web page (http://www.uow.edu.au/eng/phys/200labs/phys215/fresnel.pdf)

The separation of the images at a given lens position is related to the separation of the virtual sources like the ration of image to object distances.

Let u = image distance
let v = object distance

In the first lens position you have

\frac c {c_1} = \frac u v

At the second lens position you have
\frac c {c_2} = \frac v u

Can you work it out from there?

[physicsstudent]

Thanks, I just didnt really know where to start, but looking at that it helps a lot, as I know where to work from. Thanks again!