SCIENTIFIC SUMMARY, 
227 
PHOTOGBAPHY. 
Sun- Painting in oil colours . — Mr. Pouncy of Dorchester read a paper under 
this title at a recent meeting of the Inventors’ Institute, wherein he de- 
scribed certain improvements in his process of permanent printing in photo- 
graphy calculated to widen its sphere of application, and give it more 
practical value and interest. The sensitive medium used, is that first intro- 
duced in connection with photographic experiments, viz., bitumen of Judea, 
which is dissolved in turpentine, or benzole, with which pigments ground 
in oil are mixed so as to produce any given colour, tint or shade. Brushed 
over a thin sheet of translucent paper and dried in the dark-room it is then 
exposed to the action of natural, or some powerful artificial, light under a 
negative, until the actinic influence renders the parts to which it has had 
access in various degrees proportionate to its degrees of action insoluble in 
the original solvent, the application of which of course produces the picture. 
This is then transferred by pressure in a lithographic press to cardboard, 
canvas, wood, stone, or when ceramic colours are used by hand pressure only, 
to potter’s “ biscuit” and burnt in. Mr. Pouncy illustrated his paper with 
a great variety of specimens and went through the different manipulatory 
processes with great ease and certainty. 
The Latent Image. — We have chronicled from time to time experiments 
intended to demonstrate the true nature of the photographic image, and last 
did so in connection with the efforts of Mr. Carey Lea published in The 
British Journal of Photography. A well known French experimentalist, 
M. Davanne, writing upon the theoretical researches of the above-mentioned 
gentleman (and those of Major Eussell and MM. Poitevin and Vogel) says 
although Mr. Carey Lea has certainly cleared up one important controversial 
point, viz. the sensitiveness of iodide of silver, when pure, to the action of 
light, yet he has done nothing calculated to show definitely whether the 
latent image is due to purely chemical or to purely physical action, and adds 
“ In presence of light the iodide of silver discovers a tendency to lose a por- 
tion of the iodine, especially where it is connected with a body which is 
capable of absorbing it. It seems therefore that light not only modifies the 
physical condition of the iodide, but brings about its chemical decomposition, 
having determined the elimination of iodine. ” M. Davanne is therefore 
strongly inclined to attribute the formation of the photographic image rather 
to chemical, than to physical conditions. In connection with this subject he 
also asks those who uphold the physical theory, how they explain a curious 
fact recently made known to him by M. Magny, who having put aside a 
negative as useless, from its want of detail in the less exposed portions of the 
plate found after it had been in darkness for about six months, that well 
defined details had positively become developed by the continued slow decom- 
position of the iodide of silver. Commenting on this statement, the Paris 
correspondent of the British Journal of Photography says : “ Without 
wishing to shirk the question put by M. Davanne, I would suggest that it is 
not philosophical to assume that ‘ time can produce the same action as light,’ 
because, had that plate been exposed to light six months before it was deve- 
loped the lines and half-tones would not have appeared, and if it had been 
