630 
POPULAR SCIENCE REVIEW. 
The public have not seen^ as yet^ many of the photographs 
produced by the burning-in process ; but no doubt they will 
soon be familiar enough with these handsome and really 
important applications of the science of sun-printing. Some 
of these_, which were exhibited at the last conversazione of the 
Koyal Society were extremely pretty. When examined by 
reflected light_, they gave one simply the idea of a plate of 
white or opal-like china^ having a rather indistinct picture upon^ 
or rather slightly beneath, its surface ; but as soon as they 
were placed so that the light passed directly through them, the 
picture assumed a prominence and elegance of finish which 
could hardly have been anticipated. They are prepared by 
what is termed the hurning-in process, a method whose nature 
is expressed in its name. The photograph may be burnt into 
either glass or porcelain, but must be taken in a special 
manner. The plate must first be coated with a varnish of 
benzole, indiarubber, and collodion, and on this the sensitized 
collodion must afterwards be placed. When it has been 
exposed in the camera, the image requires to be developed, 
strengthened, and washed, according to prescribed processes 
which we have not sufficient space to describe. After these 
operations have been gone through, the plate is immersed for 
some hours in either a bath of platinum, if it be desired to 
have the picture of a greenish-black tone, or in solution of 
chloride of gold, if a golden tint is required. The image is 
now washed, either with a solution of cyanide of potassium, 
or with strong ammonia, and it is then covered with a thick 
varnish of indiarubber, and heated in a muffle."’^ The organic 
matter is thus destroyed, and a purely metallic picture remains 
upon the glass. The only thing to be done now is to vitrify 
this image, and this is effected by means of a flux of either 
borax or silica. The most important feature in connection 
with these photographs is their permanency ; the pictures are 
painted in pure metal, and this, by its vitreous envelope, is 
entirely removed from the action of the air, moisture, &c. 
Hence there is every reason to believe that such photographs 
will remain unchanged for ages ; and if such a belief be 
correct, it is most probable that a thousand years hence 
photo-portraits of our Carlyles and Tennysons will figure in 
the National Galleries of the period. 
The applications of sun-printing to science have been 
neither less important, nor less interesting, than those we 
have already touched on. Microscopy — thanks to the exertions 
of Dr. Maddox, Mr. S. Highley, and others — has profited 
largely by photography. Instruments are now employed 
which are virtually combinations of the compound micro- 
scope and the camera, and with these perfectly accurate and 
