228 PRACTICAL MICROSCOPY. 



tive. This is placed above a dry plate, the two films being 

 in contact, exposed to the light in a printing frame, and 

 the latent image developed and fixed in the usual way. 

 The best dry plates with which to prepare transparencies 

 are the collodio-albumen, developed with pyrogallic and 

 citric acids, and next to these, any good collodion plate. 

 It is a very difficult matter to prepare brilliant trans- 

 parencies upon gelatine plates ; they are so extremely 

 sensitive to light that it is next to impossible to keep the 

 high lights clear and free from fog. Very passable pictures 

 may, however, be obtained from them by giving 12 seconds' 

 exposure, under a negative of ordinary density, to the light 

 of a common bat's-wing gas-burner. 



The collodio-albumen plates, when developed with pyro- 

 gallic and citric acids, generally yield a fine blue-black 

 image; they may, however, be readily toned in a weak 

 solution of sulphide of ammonium, if requisite. 



When dry the picture should be covered with another 

 piece of clear glass the same size, the two being prevented 

 from touching by very small strips of card fastened by 

 gum to two of the edges, the whole bound round with thin 

 black paper, and the slide is completed. These pictures, 

 when laid upon white paper, should only allow the paper 

 to show through in the very highest lights, just a patch of 

 white here and there. 



For the illustration of papers read before societies, trans- 

 parencies prepared by the foregoing method may be shown 

 by the use of the Sciopticon or Triplexicon (Fig. 195), by 

 which means sufficient illumination can be obtained with 

 paraffin oil to produce a good disc 6 or 7 feet in diameter. 

 It is, of course, necessary by this method to first produce 

 an enlarged transparency of the object, which entails some 

 previous trouble, though it is less costly than that next 

 to be described. 



