"J", and enlarged details of what appears to be the
"blue stain corresponding to
figure of the glass".
[Fig. 17 credits]
I have used for this purpose blue glass . . . to permit
the eye to bear the light, and yet to intercept no more than was necessary. It is not requisite, when
coloured glass is employed, to make use of a large surface; for if the camera operation be carried
on until the proof almost solarizes, no traces can be seen in the portrait of its edges and
boundaries; but if the process is stopped at an earlier interval, there will commonly be found a
stain, corresponding to the figure of the glass. [In this daguerreotype a piece of "blue glass"
may have been nailed up in the window opening with a triangular piece of wood at top and a
rectangular piece of wood at bottom. What might be nail heads are even visible in the photo.]