SCIENTIFIC SUMMARY. 
211 
nary method of combustion, the economy of this light, when employed for 
domestic and other ordinary lighting purposes, is a subject which merits at- 
tention. The Messrs. Darker, of Lambeth, who have introduced this light, 
are engaged in the construction of burners to suit the various requirements 
of photographers. 
Another nevj Dry Process. — Dr. George Kemp, who is well known as a 
careful experimentalist in photographic science, as well as a clear writer on 
the subject, has communicated to the pages of the British Journal of Photo- 
graphy the details of a dry process which has engaged much of his attention 
and which has yielded him highly successful results. The most efficient 
preservations for dry plates, according to Dr. Kemp, are those that contain 
nitrogen. In some of these this element presents itself in such wise that 
the aggregate body may be assumed to exist as a compound of nitrogen and 
hydrogen combined with a complementary organic group, from which the 
ammonia has, in a theoretical point of view, been eliminated. In most of 
the cases, the ammonia can be liberated by familiar chemical devices, and a 
considerable number of such bodies usually designated vegetable bases were 
examined in relation to their conduct as connected with actinic reactions. 
From the experiments made. Dr. Kemp has deduced the law that all bodies 
which contain ammonia in the condition alluded to are more or less efficient 
when applied as preservative agents. From this theory he has worked out 
the following process : — On an ounce of distilled water place twenty grains 
of fresh powdered cocoa nib ) mix and allow to digest for an hour, no heat 
being employed. Filter this fluid and add two drops each of glycerine and 
glacial acetic acid. The plate, being collodionised and excited as in other 
processes, must be thoroughly washed, after which the preservative solution 
is applied and made to permeate all the fllm ; after which it is again sub- 
mitted to a thorough washing, and when dried by a strong heat is ready 
for storing or for immediate exposure. The sensitiveness is nearly, if not 
quite, as great as that of wet plates. The picture is developed by a two- 
grain solution of pyrograllic acid containing acetic acid and nitrate of silver. 
Danger of using India-Rubber, — Mr. Samuel Fry, in Photographic Neivs, 
warns photographers against employing india-rubber either as a substratum 
for the negative collodion fllm, for which it has been much used in certain 
dry processes, or as a varnish for protecting the flnished negative. It is, 
he says, a very destructible gum. Under changes of temperature or hygro- 
metric variation it loses its elasticity, becomes first brittle, and is ultimately 
reduced to a brown powder, having neither coherence nor any of the pro- 
perties of the original substance. In consequence of this instability, many 
negatives have been destroyed. 
Cracking of Negative Films. — Mr. Matthew Whiting having had some 
negatives which presented a honeycombed appearance within a few months 
after they were taken, has restored to them their original homogeneity of 
surface by pouring over them a little warm alcohol, which softened the 
varnish and caused the film to lie flat. 
The Truth of Photography versus Artistic Licence. — Mr. K. H. Bow, C.E., 
of Edinburgh, has recently been applying the theodolite to ascertain 
in a scientific manner the truth of art as displayed in certain well- 
known pictures by artists of reputation. The result of this crucial test is 
