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218 THE ADVANCE OF PHOTOGRAPHY
box was again placed upright, and brought into the camera in place of the groundglass slide, S was drawn up, and thus the plate exposed. Then the silver solution was drawn off through a stopcock, and a solution of green vitriol poured in instead ; by tilting the box this flowed over the plate and developed the picture. The development could be watched through the yellow slide S : when it was completed the picture was taken out and fixed.
Stein improved his developing box by substituting a vulcanized india-rubber vessel, easily taken out and cleaned, for the glass receiver. He also introduced the method of filling and emptying the receiver by means of a stopcock, Dubroni having employed pipettes. Both apparatus were described in detail in the Photographischen Mittheilungen, Jahrgang X., Nr. 117, 118.
Microscopic Photographs and Pigeon Post. — Some years ago jewellery and toys were offered for sale in Paris, containing small lenses in place of jewels. If these were held before the eye, small transparent pictures, some of them portraits, and other writings, were visible. These little pictures were the so-called microscopic photographs on glass. Such a picture is by no means the representation of a microscopic object, but of a large-sized object, only it is so small that a microscope is required to see it. The production of these photographs does not differ from that of others ; it only requires an instrument forming images of microscopic minuteness, and this is effected by employing small lenses of very short focal length. In using these a direct photograph is not taken, but in the first place a photographic negative is prepared with an ordinary camera from the object chosen ; after this, with the help of the small lens, microscopic positives on glass are obtained with the ordinary collodion process. These are then cut, and a small lens fastened on them, and then they are mounted in metal. Such pictures are in themselves little else than toys, which have, however, been put to a