260 
POPULAR SCIENCE REVIEW. 
Improved Method of Testing for Lime-toning Baths. — At a recent meeting of 
the South London Photographic Society — one of the most enteiprising and 
active of the numerous kindred societies — Mr. F. W. Hart described a method 
of preparing test paper, to aid in securing uniformity in the lime-toning baths, 
which has been signally successful. This description runs as follows : — First 
make a solution of ten grains of iodide of potassium in ten ounces of water, 
then make twenty-five grains of Glenfield starch into a stiff paste with a few 
drops of cold water, and add to it ten ounces of boiling water. These two 
solutions are then mixed and poured into a clean porcelain bath. Immerse 
the paper, remove, dry, and preserve it from the air. By using this the 
perplexing questions arising from the various qualities of different samples 
of lime, &c., may be avoided. If the chlorine solution prepared in its full 
quantity be tested, the paper will indicate its proper condition by turning a 
faint bluish gray ; while if it assume a decidedly blue colour, too much lune is 
present. Such is the delicacy of this test that the paper will readily demon- 
strate the presence of even one drop in ten thousand. 
Strong Negatives from Weak Ones. — The Bulletin Beige de la Photographie 
gives the following process of obtaining a strong negative from one so weak 
as to be in itself useless for the purpose of printing. The weak negative is 
placed in a camera ivith conjugate foci, a proof of it is obtained and developed 
with pyrogallic acid. The same acid with a little silver is used to give it 
intensity. A transparent positive of greater strength than the original 
negative is thus obtained, which, being placed in the back frame of the 
camera with conjugate foci, and again copied, gives a negative from which, 
when developed and fixed, positives on paper may be readily obtained. 
A Neiv Printing System. — Mr. Swan, of Newcastle, whose carbon process 
we have described in these pages, is engaged in perfecting a new system of 
printing, based upon the well-known mode of obtaining an image in relief 
by the use of gelatine, subjected, first, to the action of light under a negative, 
and secondly, to that of water. The image so obtained has the lights 
depressed and the darkened portions of the image raised. From this an 
electrotype is obtained in which the raised and depressed portions are, of 
course, reversed. This electrotype is next treated with a thick solution of 
gelatine containing Indian ink, and when this is slightly set it is printed 
from in the same way as from an ordinary copper-plate, by contact and 
pressure. It is evident that the darkened gelatine will adliere to the paper, 
and be removed from the intaglio or mould with it, when, being dark in 
degrees proportionate to its thickness, as regulated by the depth of the 
depressed surfaces, it will form an image in appearance precisely similar to 
that of Mr. Swan’s other beautiful carbon prints, but possessing many advan- 
tages in escaping the danger of cracking, &c., due to the character of the 
surface of collodion and gelatine required for the production of the latter, 
and in the superior speed, certainty, and cheapness of the former’s produc- 
tion. This is a most important and promising step, which we hail with con- 
siderable pleasure and hope. 
Pfr. Duchennds Auto-Photographic Process. — M. Duchenne, of Boulogne, 
long since conceived the idea of representing photographically microscopic 
studies of the nervous system, so as to compare the normal with the patho- 
logical condition. After many attempts, dating back some years, a consider- 
