358 De la Rue and Celestial Photography. [ July, 
In fine, there are few branches of British out-of-door geology 
in which the student will find more to interest him than in the 
story of our old volcanoes, or where, by diligent work, he will be 
more likely to discover new facts, and thus add to the treasures of 
the science. The apparent repulsiveness of the subject will soon 
disappear as he enters fully into his self-appointed task ; and even if 
he should content himself with simply treading in the path that has 
been laid out for him by the laborious footsteps of earlier observers, 
he will not have spent a week or two in the pursuit without gain- 
ing new bodily vigour, and carrying away with him many pleasant 
memories of the rocks, quarries, and hill-sides among which he 
was at work. 
VI. DE LA RUE AND CELESTIAL PHOTOGRAPHY. 
It is about twenty-seven years since we were told of a remarkable 
discovery, made by a Frenchman, of a process by which external 
objects were made to delineate themselves on prepared metal 
plates, placed in a camera-obscura, with a perfection of detail 
and a delicacy of delineation which had never been approached by 
the human hand. The pictures so produced by Daguerre were 
seen and admired. ‘The world of Science, once awakened to the 
fact, that the Sun’s rays could be made to copy, on prepared tablets, 
the objects which they illuminated, went busily to work investi- 
gating the curious phenomena involved in the art of Photography. 
It must not be forgotten that Wedgwood, assisted by Davy, pro- 
duced wnstable photographic pictures in 1805.* Mr. H. Fox 
Talbot, soon after the announcement of Daguerre’s discovery, 
produced his “ Photogenic” drawings, speedily followed by his 
beautiful “calotype” pictures. Sir John Herschel investigated, 
with much industry and skill, the chemical changes produced upon 
organic and inorganic substances by solar agencies; and Mr. 
Robert Hunt published, in rapid succession, his discoveries of the 
developing power of the proto-sulphate of iron, of the influence of 
the chemical rays in accelerating the germination of seeds, his 
“chromatype,” and other processes for producing photographic 
pictures. Beyond this, at the second meeting of the British 
Association at York, in 1844, this photographer showed that the 
chemical changes produced by the sun’s rays were not due to their 
luminous power, but were the consequence of dark radiations, for 
which principle or power he proposed the name of Acrinism—a 
term which has been generally adopted. . 
These researches appeared to confirm the results obtained 
* « Journal of the Royal Institution,’ vol. i. 
