152 Trans. Acad. Sci. of St. Loute. 



has not been tried, but some of the best results were obtained 

 from the last of a box of plates which were all exposed at 

 the same time, and which were not all used until nine days 

 had elapsed. This method of treatment is advantageous be- 

 cause it is difficult to prevent light from the discharge from 

 striking the plate during the electrical exposure, and a great 

 over-exposure renders the plate more manageable in the 

 subsequent treatment. It darkens much more slowly in the 

 developing bath than when slightly light-struck. 



The plate is put in position with the medal resting upon it. 

 The capacity described gives a rather rapid sequence of small 

 sparks, which may be made 15 cm. in length. At each dis- 

 charge between the knobs of the machine, a discharge occurs 

 on the film around and under the medal. This exposure may 

 be from four to ten minutes. A much longer exposure re- 

 verses the picture and gives a positive. The exposure should 

 be in a darkened room, and the light from the spark should 

 be kept from the plate by a screen. Light falling on the 

 plate while the electrical action is taking place, counteracts 

 the electrical action in a very remarkable way. This may be 

 shown by partly closing the blinds of a window ten or fifteen 

 feet away, forming thus a vertical slit a foot in width. The 

 other blinds are to be wholly closed. A book set up so as to 

 shade half of the plate yields results such as are shown in 

 Fig. 1. This print is of course a positive from the original 

 negative. 



It is therefore evident that the time of exposure depends 

 somewhat on the diffuse illumination in the room. A very 

 dark room is not necessary. 



It is also found that if the plate be exposed to light for a 

 day or more after the electrical exposure, a similar counter- 

 acting effect is produced. In this way the picture may even 

 be reversed and develop as a positive. 



In developing the picture a cool and rather weak hydro- 

 chinone developer leaves nothing to be desired. The room 

 should not be too dark during this operation. The best con- 

 ditions are to be found in an ordinary dark room, lighted by 

 a single incandescent lamp. The light should be five or six 

 feet away, and any tendency to fog is remedied by taking the 



