It seems you would basically want an inverted copy of the blue channel  
with white regions being transparent. There are dozens of ways to  
accomplish this. I would propose the following:

Decompose you image to RGB components.
In the new image, hide the red and green layers.
Add a layermask to the blue layer, initialized to a "Grayscale copy of  
the layer".
Fill the blue layer with black.

OPTIONAL: Most orthochromatic film would respond to some extent to  
green, so repeat the last two steps on the green layer, make it  
visible, and set its opacity to a relatively low value (0-30%).

This method is a bit crude, but should provide something not entirely  
unsuitable. A more accurate approach would probably entail applying  
appropriate curves to the different color channels.

Quoting norman <[EMAIL PROTECTED]>:

> I would like to be able to produce a black and white negative image from
> an ordinary colour image. The effect I would like to produce would be to
> imitate what one would get if you took a colour, positive transparency
> and made a contact print on to either orthographic or lithographic film.
> Thus, blue would be black and red would be clear and transparent. Have
> you any suggestions how to approach this using the Gimp, please.

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