[Hologram]

Now, the thing that has me wondering is whether or not you can just leave the emulsion on the mirror during processing & for viewing. There seems to be some disagreement, or at least a lack of clear concensus on this.

You can leave the emulsion on the mirror for processing, Chris - but not for viewing. To what extent this is doable practically is another matter (you've to struggle with two opposing issues: on the one hand the emulsion must remain on the mirror/glass plate during processing and on the other hand, you have to be able to remove the developed layer from the mirror without damaging it subsequently).

J.S. Friedman in the History of Color Photography clearly states that the mirror must be present for viewing, and that this is of course difficult with mercury.

I've Just had a look into Friedman: "...the illumination of these plates must be in accordance with the taking scheme; that is, with a mirror in contact with the print so that standing waves will be reformed. For this reason, the plates can be viewed only at a certain critical angle."

Friedman is just plain wrong about this. Obviously, he is confusing the mirror (reflector, mercury layer etc.) used during recording with the glass wedge, which the Lippmann photograph used to be optically contacted to for better viewing. The 10° wedge simply serves to shift the viewing angle away from the specular reflection of the glass surface.