NATURAL PHILOSOPHY. 187 



IMPROVEMENTS IX PHOTOGRAPHY. 



Paper Pliotoqraplis. Mr. Peter Fry adopts a simple mode of man- 

 ipulation, offering many advantages to the traveller. Papers are 

 prepared with very weak solutions, so that they are covered with a 

 pure iodide of silver, to which the slightest addition of nitrate of sil- 

 ver will give sensibility. Papers might be iodized in England pre- 

 viously to~a three months' travel. A small quantity of a very strong 

 solution of nitrate of silver, and the same of gallic acid, are the only 

 exciting agents required. In the morning, as many pieces of paper 

 as are thought to be necessary for the day, are rendered sensitive. A 

 few drops of the strong solution of the silver salt are added to some 

 pure water, and they are washed with this on one surface, and placed 

 in their frames. The pictures are obtained, and they are developed 

 on the traveller's return at night with very little trouble. Thus a 

 very small addition to the ordinary luggage is required, and at any 

 place all the operations can be very easily performed. Sir William 

 Newton has adopted the practice of copying the negatives on a paper 

 similar to that on which they were obtained. His positive paper is 

 prepared with from seven to ten grains of nitrate of silver to one 

 ounce of distilled water the iodide of potassium being in equivalent 

 proportions. It is excited by the aceto-nitrate solution, and exposed 

 to light for from five seconds to half a minute, and developed by gal- 

 lic acid. By this the process of copying is exceedingly facilitated, 

 and in almost any state of weather it can be carried on, and even by 

 good artificial light successfully pursued. 



Blanquart Everard and Gustave Le Gray have for some time 

 adopted a process not very dissimilar to that of Sir William Newton, 

 in its general character. Their positive pictures are of a very uniform 

 tint, and of a fine black or dark violet color, which adds much to their 

 beauty. This color is produced by the following method, according 

 to M. Le Gray. The paper for the positives is prepared with muriate 

 of ammonia and nitrate of silver. When a blue-black is desired in 

 the darkest parts, the whites of the picture are allowed to assume a 

 clear violet tint before it is removed from the sunlight. For a pure 

 black a still more decided violet color should be obtained ; and for an 

 olive black the whites should even pass into a sepia tone. The pic- 

 tures are first washed in ordinary water, to remove all the free nitrate 

 of silver, or to convert it into a chloride. It is then placed in a bath, 

 prepared in the following manner : Chloride of gold, 1 grain ; dis- 

 tilled water, 1000 grains; hydrochloric acid, 25 grains. This solution 

 clears all the white parts, and the sepia or olive tints pass to black. 

 When the blacks are perfectly clear, and all the details of the nega- 

 tive developed, the picture is well and quickly washed in several 

 waters, to remove all traces of acid. This is very important, since 

 any adhering acid occasions the decomposition of the hyposulphite of 

 soda, and by precipitating sulphur destroys the picture. A little am- 

 monia, added to the water, effectually neutralizes the acid. It is then 



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