SCIENTIFIC SUMMAEY. 
261 
able amount of success has been obtained, as was proved by the specimens of 
the process recently presented to the Academic des Sciences. Dr. Duchenne’s 
paper (condensed) says : — “ The plates which I now present to the Academie 
show, in comparison with the normal state, the microscopic anatonio-pathology 
of the spinal roots in the progressive locomotive ataxy. The great number 
of the figures, when compared with the original microscopic preparations, 
show that they are rendered with that exactitude which the hand of man 
could but rarely equal. The price of printing each plate will not exceed six 
centimes. If we consider that these figures are not changeable, like those 
produced by photographic printing, we shall comprehend how much photo- 
autogTaj)hy, applied to microscopic works, interests the progress of science. 
The process employed is similar to that of Colonel James in England. A 
positive is printed upon bichromated gum or gelatine paper, and covered 
with a greasy ink, which only adheres to such portions of the print as are 
insoluble after washing, which is a delicate operation, requuing frequent 
repetition and considerable experience and care. This is then transferred, as 
in ordinary autography, to a metallic plate or prepared stone, into the body 
of which it penetrates without the interposition of any foreign body. From 
this 1,500 or more excellent proofs may be obtained.” The process, however, 
to which Dr. Duchenne owes his ultimate success is that of Captain Ban, a 
French gentleman, attached to the stafi* of the Minister of War, and which 
is thus described : — 
1st. Cover the surface of a zinc plate or lithographic stone with a thin film 
of biclmomate of potassa mixed with sufficient sugar-candy to give a smooth 
polished surface. 
2nd. Expose under a positive so as to obtain a negative. 
3rd. Wash away with a sponge, lightly applied, to remove the soluble or 
dark portions of the bichromate (those not exposed to the light’s action). 
4th. Apply lithographic ink, which enters deeply into the bare portions of 
the surface and covers the blackened parts ; then clean the surface as with 
an engTaving. 
5th. Dissolve the exposed bichromate with water slightly acidulated, which 
will not disturb the lithographic ink. 
6th. W ash well under a tap with running water so as to carry off the 
bichromate of potassa and leave only the design in greasy ink, which has 
penetrated into the substance of the stone. 
The ]3late is afterwards submitted to the usual lithographic treatment. 
The Use of Cyanide of Potassium . — So many accidents have occurred, and 
do continually occur, through the use of this dangerous chemical, some of 
which have been fatal, and all of which must be serious, that it may be as 
vvell to remmd those photographers who have been in the habit of using it to 
cleanse their hands from silver stains, that if they promptly wash them in 
water, using a piece of pumice stone, such a highly objectionable process of 
cleansing need not be adopted. 
Astronomical Photography. — M. Faye, in some memoirs presented to the 
Paris Academie des Sciences, endeavours to point out some errors which the 
present system of observation produces in the astronomical determination of 
the hour, and shows that the only remedy against such is to be found in the 
adoption of electric telegraphy and photography. M. Faye says “ The possi- 
