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sun from behind clouds, of which the camera takes no notice, 
and on the other hand, there may be effects in nature 
invisible to us, but which act like the invisible rays of the 
spectrum with great chemical intensity. 
The following photographs were exhibited to illustrate 
the above remarks : — 
No. 1. Showing white lines round figures, and' chair in 
carte de visite portrait. 
No. 2. Showing white line round church tower and yew 
tree. 
No. 3. Showing white line faintly above the hill, also 
the increased depth of shade at the extreme top of it. 
No. 4. Showing intense white line dividing landscape 
from sky, also the railings on bridge against the sky showing 
intense white, whilst those against the trees are black. 
No. 5. Curious compound of positive and negative, the 
house, road, trees, &c., in foreground being positive, and 
correct, the sky black, and the distant trees and buildings 
white. 
No. 6. Bridge and stream, Fountains Abbey. Showing 
the foliage around the opening to sky traced in pure white, 
the other part of photograph being natural. 
No. 7. Showing the blurring of dark tracery when light 
streams through a window. 
No. 8. Showing one of the distant hills reversed, appear- 
ing light against a dark sky, the other mountains appearing 
natural. 
Mr. Dancer remarked that several years since he had 
attempted to explain the marginal appearances which some 
photographic negatives exhibit, by supposing that the force 
which governed the deposit forming the picture was more 
energetic at the abrupt boundary line between a light and 
dark object than on other parts of the exposed sensitive 
plate where there was less contrast,, this intensity of force 
bearing some analogy to that which is observed on the edges 
