This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
By A. E. SWOYER
When the photographer wishes to make an enlarge 'print from a small negative, he arranges a suitable light and condensers back of the negative and by means of a lens projects the resultant image upon a sheet of sensitive paper. Owing to the comparative weakness of the light, however, it is necessary either to use bromide paper or some of the faster brands of de-
Ill: Fig .1 Fig.2 Fig.3
An Ordinary Post-Card Projector Used Back of a Camera to Illuminate a Photograph Which is Enlarged on a Plate to Make a Negative Instead of a Print veloping-out paper. If a more artistic medium is desired, a glass positive must first be made and enlarged to produce a negative from which the final prints will be made by contact. This process is somewhat clumsy and expensive, for if any retouching or doctoring is to be done, it must be upon a glass surface, either that of the two negatives or of the intermediate positive. As all of this work is done by transmitted light, there is the loss of fine detail common to all enlargements.
The difficulties incident to this process may be done away with by the use of a modification of the popular post-card projector; the alteration consisting simply in the substitution of a better lens for the cheap plate glass with which such instruments are usually fitted.
A contact print, preferably on glossy paper, ferrotyped, is made from the original negative by contact in the usual way; this is then placed in the modified projector and the image thrown upon a sensitive plate of the desired size. , After a brief exposure, development will show an enlarged negative having every quality of the original.
The advantages of this process are obvious. In the first place, the comparative cheapness of the apparatus is a factor; in the second, the intermediate glass positive is eliminated, the print which is substituted for it providing a much better medium for retouching, faking or printing in. Transparent water colors in the less actinic shades may be used upon this print to control the final result, and if spoiled, it may be replaced at a negligible cost.
At first glance, it would appear as if this method were simply a form of photographic copying-; it is, in fact, the reverse. For in copying any object with a camera, the sensitive medium is behind the lens and the object to be copied is in front, and the size of the copy is therefore limited both by that of the camera and by its bellows draw. In the reflection process, the object to be copied is back of the lens and the sensitive medium is in front; as large a copy can be made with a small camera as with an eight by ten. It is really more convenient to work with a short-focus lens and a camera of limited bel~ lows extension; the nearer the lens is to the back of the camera the larger will be the projected image.
The diagram (Fig. 1) shows that the size of the object to be enlarged does not depend upon the focal length of the lens used, as in ordinary enlarging, but simply upon the size of the opening in the front of the projector. The dotted lines are drawn from the edges of the card to be projected through the lens. Figure 2 is a sketch of a projector with the lens tube removed, so that it may be used with a camera as shown in Fig:. 3.