300 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



plate, and this destroys the clearness of the design. If the operator 

 has failed in his first experiment, let him immediately commence 

 Avith another plate; correcting the second trial by the first he will 

 seldom fail to reproduce a good photograph. 



Fourth (jpenifion.— The apparatus required in this operation is 

 represented by figure 10. A is the lid of the box ; B, a blackboard with 



grooves to receive the plate; C, cup con- 

 taining a little mercury (J) : D. spirit 

 lamp; F, thermometer; G, glass 

 through which to inspect the operation; 

 H, tablet as removed from the camera; 

 I, stand for the spirit lamp. All the 

 interior of this apparatus should be cov- 

 ered with hard, black varnish. The 

 board and the affixed plate being with- 

 drawn from the camera are placed at an 

 angle of about 45° within this box, the 

 tablet with the picture downward, so 

 that it may be seen through the glass G. 

 The box being carefully closed, the 

 spirit lamp is to be lighted and placed 

 under the cup containing the mercury. 

 The heat is to be applied until the ther- 

 mometer, the bulb of which is covered 

 with the mercury, indicates a tempera- 

 ture of 60° C. (140° F.). The lamp is then withdrawn, and if the 

 thermometer has risen rapidly, it Avill continue to rise without the 

 aid of the lamp, but the elevation ought not to be allowed to exceed 

 75° C. (167° F.). 



After a few minutes the image of nature nnpressed, but now in- 

 visible, on the plate begins to appear. The operator assures himself 

 of the progress of this development by examining the picture through 

 the glass G, by a taper, taking care that the rays do not fall too 

 strongly on the plate and injure the nascent images. The opera- 

 tion is continued till the thermometer sinks to 45° C. (113° F.). 

 When the objects have been strongly illuminated, or when the plate 

 has been kept in the camera too long, it will be found that this 

 operation is completed before the thermometer has fallen to 55° C. 

 (131° F.). This is, however, always known by observing the sketch 

 through the glass. 



After each operation the apparatus is carefully cleaned in every 

 part, and in particular the strips of metal which hold the plate 

 are well rubbed with pumice and water to remove the adhering 



