40 Prof. Rood on some Experiments connected 



1. A crumpled sheet of tinfoil was photographed, and from 

 the negative, prints were taken by the " ammonia-nitrate pro- 

 cess/' which were toned to the so-called black of the photo- 

 graphers. This furnished dark paper upon whose surface was 

 an accurate drawing of the irregularities characteristic of me- 

 tallic foil ; the surface of the paper was of course wholly with- 

 out lustre. 



(a) Upon combining, in black or white fields, a square inch 

 of one of those photographs with the above-mentioned yellow 

 paper, and shading the photograph a little, a representation of 

 gold was obtained but little inferior to that given by the use of 

 the real tinfoil. 



(b) This photographic paper in combination with orange paper 

 (1st circle, orange) made an imitation of metallic copper. 



(c) The ultramarine paper in combination with tlje photo- 

 graph of tinfoil gave a striking imitation of foliated graphite. 

 The blue colour is perceived much less than would be expected. 



2. The surface of a plate of brass 1 inch square was polished, 

 and then rather heavily scratched by a coarse file. Into the 

 scratches a small amount of yellow or white oil paint was 

 rubbed, and upon this prepared surface dark grey or black 

 paper was laid, and the whole submitted to the action of a press 

 as in copper-plate printing. By this means a drawing of a 

 scratched metallic surface was transferred to paper. These 

 markings serve also to enable the observer much more easily 

 to direct his attention simultaneously to the two impressions 

 presented. 



(a) Upon combining dark grey paper (black and white scale, 

 Nos. 18, 19, 20) prepared in this way with the above-mentioned 

 yellow paper, the appearance of a polished, scratched plate of 

 gold was obtained. 



(b) When these dark prepared papers were combined with 

 yellow paper coloured by gamboge (yellow and black scale, No. 9), 

 the appearance and lustre of brass were obtained. 



According to Dove's theory the darker surface in the stereo- 

 scope represents the dispersed light, the brighter, that regularly 

 reflected. As the polish of a metallic surface is proportional to 

 the smallness in amount of the light it disperses, we should be 

 led to expect that by varying the shade of the black paper, we 

 should be able to alter the apparent degree of polish of these 

 imitated metallic surfaces. 



This is the case: yellow paper (1st circle, orange-yellow 

 No. 4), in combination with black (No. 21), gives the idea of a 

 very highly polished golden surface ; as we descend in the scale, 

 the lustre and resemblance to polished metal regularly dimi- 

 nishes till at grey, No. 8, almost no effect like gold is to be 

 perceived. 



