Everybody knows that in B&W photography an yellow filter improves the rendering of clouds making them clearer compared the blue sky. But very often the shadows on your subject are highly lit by blue light and a normal yellow filter will make them too much dark to show details in those areas. This "écran degradée" is the perfect answer to this problem. You can slide it and control the amount of yellow filtering you put on the upper part of your picture. Last but not least: check out the box !!
Better than this only Photoshop.

I have here a demonstration. The sky was deep blue. I shot it with the above featured Zeiss Ikon Contaflex reflex camera. Both negatives were enlarged together to be sure the expostion and development would be the same.



This is a view of the access to Chantilly castle in France. Note that the filtration effect is much stronger on the upper part of the picture. Note as well that, as it always happens in bright sun days when we use a yellow filter, that the shadows are darker. This is due to the fact that not receiving direct white sun light, the shadows are lit by the blue part of the sky. We can see details in the shadows without filter because this blue light is well reflected to our eyes as well. But the filter cuts off the blue and then the shadows (as the sky) become really dark.