nnexed letter from my brother-in-law, Mr. John
Stewart, of Pau, will much oblige me. The utility of this mode of
reproduction seems indisputable. In reference to its concluding paragraph,
I will only add, that the _publication_ of concentrated microscopic
editions of works of reference--maps, atlases, logarithmic tables, or the
concentration for pocket use of private notes and MSS., &c., &c., and
innumerable other similar applications--is brought within the reach of any
one who possesses a small achromatic object-glass of an inch or an inch and
a half in diameter, and a brass tube, with slides before and behind the
lens of a fitting diameter to receive the plate or plates to be operated
upon,--central or nearly central rays only being required. The details are
too obvious to need mention.--I am, &c.

"J. F. W. HERSCHEL.

* * * * *

"Pau, June 11.

"Dear Herschel.--I sent you some time ago a few small-sized studies of
animals from the life, singly and in flocks, upon collodionised glass. The
great rapidity of exposition required for such subjects, being but the
fraction of a second, together with the very considerable depth and harmony
obtained, gave me reason to hope that ere this I should have been able to
produce microscopic pictures of animated objects. For the present, I have
been interrupted. Meantime, one of my friends here, Mr. Heilmann, following
the same pursuit, has lighted on an ingenious method of taking from glass
negatives positive impressions of different dimensions, and with all the
delicate minuteness which the negative may possess. This discovery is
likely, I think, to extend the resources and the application of
photography,--and with some modifications, which I will explain, to
increase the power of reproduction to an almost unlimited amount. The plan
is as follows:--The negative to be reproduced is placed in a slider at one
end (_a_) of a camera or other box, constructed to exclude the light
throughout. The surface prepared for the reception of the positive--whether
albumen, collodion, or paper--is placed in another slider, as usual, at the
opposite extremity (_c_) of the box, and intermediately between the two
extremities (at _b_) is placed a lens. The negative at _a_ is presented to
the light of the sky, care being taken that no rays enter the box but those
traversing the partly transparent negative. These rays are received and
directed by the lens at _b_ upon the