98 Scientific Proceedings, Royal Duhlin Society. 



toned matter iu this manner, the areas of maximum illumination are those 

 which are illuminated by the incident light plus the light reflected by the 

 white paper. The areas in contact with the black portions of the original are 

 only illuminated by the incident light pins tlie small proportion reflected by 

 the black ink. The incident light greatly exceeds the reflected light in 

 amount; and the difference in illumination relied upon to produce tlie 

 necessary contrast does not, in most cases, exceed 5 per cent. With this 

 small margin of additional illumination it is found possible, however, to 

 obtain a contrast in the negative amounting to as much as 40 or 50 per cent. 

 Once this is obtained, the negative may iu turn be used to produce a 

 transparency in whicli tlie contrast is further increased by a second 

 application of the same treatment. Workable lantern-slides are obtained 

 in this manner (PL VIII,, fig. 5) ; but much better results, comparable to the 

 best lantern-slides obtained with the camera, are secured by two additional 

 reversals, to which the same principle is applied (PI. YIII., figs. 4 and 6). 



Instead of making a number of successive reversals, tlie fog may be 

 eliminated by reduction and subsequent intensification. Howard Farmer's 

 reducer (potassium ferricyanide and hyposulphite of soda) dissolves away 

 the fog more than the full-tone if sufiiciently concentrated. The negative is 

 intensified with mercuric chloride and silver nitrate. 



The best results are obtained with slow plates of the " photomechanical " 

 class, and the developer used was the following : — 



No. 1 solution. 



Hydroquinone, . . .80 grains. 



Potassium metabisulphite, . 120 grains. 



Potassium bromide, . . 10 grains. 



Water, &c., . . . .10 ounces. 



No. 2 solution. 

 Caustic potash, . . . 200 grains. 



Water, &c., . . . .10 ounces. 



Equal parts of both solutions are mixed. 



The best fixing agent is potassium cyanide, on account of its solving 

 action on thinly deposited silver. But I used the ordinary " hypo " batli. 



Other Applications of the Process. — Excellent paper negatives are obtained 

 with rapid bromide papers, and also with gas-light papers.' On printing 

 positives from them in the ordinary way, the grain of tlie paper negative 



' See foot-note, p. 97. 



