The image can be projected with background light: If the glass image can be viewed with background light, it is a film positive.

The image is recognizable as negative with background light: Ambrotypies (glass negatives) have been dark lacquered or underlaid with dark cardboard, velvet or lacquered metal to appear as positives. That is why they have been treated as positives here. In transmitted light, however, they are partly recognizable as negatives.