The only way I know of is to scan as a reflective document. I use "professional mode". Put your negs in their "print file" or whatever sleeves on the bed of the scanner and close the lid. Set Document type to "Reflective" and Auto Exposure type to "Document". I scan at 300dpi or below, otherwise it just takes too long. Invert, desaturate and do levels in photoshop. I end up with something like the attached jpg. Very dusty, but somewhat useful. The other option, if you have a digital camera, is to put them on a light table and actually photograph them. Then invert etc. in photoshop. A little more labor intensive but, in some ways, better.