Looks like George was pretty close. If you are contact printing and don't need to see the neg well and can tolerate increased exposure, put the neg under a diffusion material directly. An example of this would be to use white plexiglass directly above the neg (between the neg and the light source). This gives a relatively good amount of diffusion. I don't think Newton can make his little rings in diffuse light.
The same thing works with diffusion enlarging and coldlight. Make the diffusor the top part of a 'glass' neg holder. The bottom can be regular glass.