Should any one feel desirous to test more minutely the effect produced by a bevel top bath, I would suggest to them to place a frame, so constructed as to hold three sixth size plates, and fit it to the top of the bath, and so arrange it with openings that the plates may be placed, one at the nearest point of the mercury, the second midway, and the third to the greatest distance, and by placing the plates over at one and the same time, the experimenter will be enabled to judge if there exists a difference in the developing.

In speaking of the above, reference is had to baths to the ordinary heights used by operators.
We will now proceed to examine the effect produced by mercurial vapor upon the plate at different lengths of exposure.

In some investigations which I have made upon the appearance of the Daguerreotype impressions when developed over mercury at 90 deg.

C.
(194 deg.

F.), the following was the result.