122 THE PROGRESS OF PHOTOGRAPHY. 
glass. The spirits have sought to fix these floating 
images; they have made a subtle matter by means of 
which a picture is formed in the twinkling of an eye. 
They coat a piece of canvas with this matter, and place 
it in front of the object to be taken. The first effect of 
this cloth is similar to that of a mirror, but, by means of 
its viscous nature, the prepared canvas, as is not the case 
with the mirror, retains a fac simile of the image. The 
mirror represents images faithfully, but retains none. 
Our canvas reflects them no less faithfully, but retains 
them all. This impression of the image is instantaneous. 
The canvas is then removed and deposited in a dark 
place. An hour later the impression is dry, and you 
havea picturein that no art can imitate its truthfulness.” 
This was, of course, several years before Scheele’s ex- 
periments, and was purely the creation of de la Roche’s 
imagination, yet it reads to-day like a remarkable 
prophecy, and surely may have stimulated_the efforts of 
those who ultimately solved the problems of sun printing. 
The early years of the nineteenth century were marked 
by the experiments of Wedgwood (one of the great 
English doctors) and of Sir Humphrey Davy, both of 
whom endeavored to secure the pictures formed within a 
camera, but without success. Davy discovered that 
chloride of silver was much more sensitive than the ni- 
trate, and by using the concentrated light of the solar 
microscope he obtained images of small objects upon pa- 
per which had been coated with chloride of silver. 
How to secure these images presented a new problem, 
which Davy did nct live tosolve. He wrote: ‘‘ Nothing 
but a method of preventing the unshaded parts of the 
_delineations from being colored by exposure to the day is 
wanting to render this process as useful as it is.elegant.”’ 
The experiments of Nicéphore Niepce, with the bitu- 
men process of contact printing, and the action of light 
upon silver, bitumen and other substances, are inter- 
72 
