_Dr. Mansell's Process._--DR. MANSELL'S lucid and very practical paper on
the calotype process in "N. & Q." must, I am sure, be of the greatest
service to photographers in general; and as one of the many I am
irresistibly tempted to offer my sincere and hearty {183} thanks to him for
the truly valuable hints it contains. If DR. MANSELL will give the
rationale of the necessity of not allowing a longer time than absolutely
required for the soaking out the now injurious iodide of potassium, set
free by the deposit of the iodide of silver; and also, an explanation of
the cause of that part of the iodized papers which takes the longest time
in drying being weaker than that part which had been more hastily dried,
the learned Doctor will still be adding to our present account of
obligation to him.