"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 sensitive surface at _c_, and the impression of the negative is there produced with a rapidity proportioned to the light admitted, and the sensibility of the surface presented. By varying the distances between _a_ and _c_, and _c_ and _b_, any dimension required may be given to the positive impression. Thus, from a medium-sized negative, I have obtained negatives four times larger than the original, and other impressions reduced thirty times, capable of figuring on a watch-glass, brooch, or ring.