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POPULAR SCIENCE REVIEW. 
that is that the number of planes M. Claudet could put successively in and 
out of focus would in too great a degree depend upon the length of the 
exposure. For instance, in taking the portraits of children, when there is 
usually only time enough to remove and replace the cap of the lens, the back 
lens could not well be moved at all, while in dull weather the rack could 
hardly be moved slowly enough, and the intervals between the movements 
of the back lens would probably have to be seriously lengthened. A process 
so variable in its results, and which is so dependant upon numerous and 
ordinary chances for its success, cannot be a good one, nor can we agree with 
M. Claudet in regarding it as “ the greatest improvement which will have 
been introduced in photography.” But supposing it really did equalize the 
definition of various planes of distance. Is this a desirable result ? Does 
the human eye, which M. Claudet accepts as the ideal of what the photo- 
graphic lens and camera should be — does this see all objects or planes, near 
and remote, with equal distinctness ? No one can imagine it does so, and 
therefore our best artists are careful to give to these planes their relative 
degrees of distinctness in accordance with the aim of art and the facts of 
nature, as demonstrated by science. Upon this principle should photographic 
lenses be manufactured, and upon this principle should the operator work 
when focussing the images in his camera. M. Claudet is a veteran photo- 
grapher, and was one of our first Daguerrean portraitists. Many valuable 
hints and suggestions have emanated from him at different times, but the 
long paper he read before this year’s meeting of the British Association is 
an exception. 
Action of Light upon Iodide of Lead. — M. Schonbein has published some 
conclusions based upon experiments with iodide of lead subjected to the 
action of light, which are interesting and suggestive. When perfectly dry 
the iodide remained undecomposed, but it decomposed when kept constantly 
moist and in contact with air. Mixed with starch or tincture of guaicum it 
remains unaltered in vacuo, but on the admission of air is at once affected, 
the blue coloration showing the liberation of iodine. In the decomposition 
of pure moist iodide of lead by light it grows lighter in colour, loses iodine, 
and gives rise to the formation of peroxide and carbonate of lead. Any 
substance having a tendency to unite with iodine will assist in determining 
the decomposition of iodide of lead in sunlight. A mixture of starch-paste 
and iodide of lead is not acted upon by diffused light although strong direct 
light affects it immediately. It is singular to find the iodide of lead under 
the action of light so similar to the iodide of silver. Ghloride of lead 
undergoes no change from light. 
The New TJse of Peroxide of Hydrogen and Chlorine Water. — In our last 
photographic summary we called attention to Dr. Angus Smith’s discovery as 
to the use of peroxide of hydrogen in eliminating from photographs the 
destructive hyposulphite. Since then this new process has foimd opponents. 
Mr. F. W. Hart suggests that the hydrochloric acid used in its preparation, 
and the fact of its decomposing when in contact with gold or silver, render 
it a very unsuitable agent. In reply, Mr. Dawson affirms as the result of 
experiments that the preparation does not decompose when in contact with a 
silver print, and that the trace of hydrochloric acid is too trifling to exert any 
injurious influence. Dr. J. Emerson Keynolds, after using some extremely 
