624 
POPULAE SCIENCE EEVIEW. 
The foregoing is a very elementary sketch of the photographic 
process. The earlier photographs^ or_, as they were styled 
(after their inventor^ M. Daguerre), daguerreotypes, were 
done upon copper plates which had been previously silvered. 
These having been exposed in the camera — an instrument 
in which an accurate picture of surrounding objects is thrown 
upon the introduced plate — were afterwards fixed, somewhat 
in the manner we have above described. The daguerreotype 
gave way to a cheaper process, in which a film of collodion 
(gun-cotton dissolved in ether) deposited upon glass was 
made to receive the sensitized solution; and this, in its turn, 
has almost given way (as a picture) to the paper process. 
In all the various methods of photography now employed in 
portraiture, the principle is the same : a salt of silver, more 
frequently the iodide, is exposed in a camera, and an image 
thrown upon it by the agency of the latter. The light falling 
upon diferent parts of the plate with different intensities, 
produces a decomposition of the silver, corresponding to 
the lights and shades of the picture. The plate is then 
removed to a dark room (to prevent the further action of 
light), developed by special solutions, and fixed. Where, 
however, paper is the medium employed, there is some 
greater difficulty in producing the requmed result. Sensitized 
paper is scarcely as sensitive as either the collodion film on 
glass or Mr. Daguerrre^s plate, and if a picture were taken 
with it in the ordinary manner, it would be necessary to have 
it exposed for a very considerable time. Hence, paper can- 
not be employed in this manner for portraits. How then, it 
will be asked, are our cartes executed? We will explain; 
for the explanation leads us to a new portion of our subject, — 
the respective meanings of the terms jjositive and negative. 
The ordinary picture, which is produced in the case of the 
collodion film on glass, is a positive ; that is to say, it is a true 
delineation of the object it portrays, light representing light, 
and shade, shade ; in point of fact, it is like any picture. In 
the negative, the state of things is reversed ; that which is 
shade in the positive is light here, and vice versa. In fact, a 
negative is a positive in which all the shades have been 
reversed. From this it follows, that if we place a piece of 
photographic paper beneath a negative, and expose them thus 
to the sun^s rays, we shall have a poMtive printed upon the 
paper. This is just what the photographer effects in pro- 
ducing a carte portrait. Without entering minutely into the 
method of producing a negative or a positive, we may state 
generally that in the one case, all the decomposed, and in the 
other, the undecomposed, silver is removed. In taking a carte, 
therefore, the photographer first procures a negative picture 
