All these must be placed in a line, so that the best part of the light, the center of the condenser, and the lens are of equal height.
The method of working is as follows: The lamp, B, is placed at such a distance from the condenser that the rays come to a focus and enter the lens; the negative is then placed in the frame, the focus obtained, and the size of reduction adjusted by moving the camera nearer to or further from the condenser and negative. In doing this no attention need be paid to the light properly covering the field, as that cannot be adjusted while the negative is in its place. When the size and focus are obtained, remove the negative, and carefully move the lamp till it illuminates the ground glass equally all over, by a disk of light free from color.
The negative can then be replaced, and no further adjustment will be needed for any further reproduction of the same size.
There is one point that requires attention: The lens used in the camera should be a doublet of about 6 inch focus (in reproducing 8½ × 6½ or smaller sizes), and the stop used must not be a very small one, not less than ½ inch diameter. If a smaller stop is used, an even disk of light is not obtained, but ample definition is obtainable with the size stop mentioned.
In the arrangement described, a single lens is used for the condenser, not because it is better than a double one, as is general for such purposes, but because it is quite sufficient for the purpose. Of course, a large condenser is both expensive and cumbersome. There is, therefore, no advantage in using a combination if a single lens will answer.
In reproducing lantern pictures from half-plate negatives, the time required on my lantern plates is from two to four minutes, using 6 inch condenser. For whole plate negatives, from two to six minutes with a 9 inch condenser. In working in this way it is easy to be developing one picture while exposing another.
The condenser must be of such a size that it will cover the plate from corner to corner. The best part of an 8½ × 6½ negative will be covered by a 9 inch condenser, and a 6½ × 4¾ by a 6 inch condenser.
With this arrangement it will be easy to reproduce from half or whole plate negatives or any intermediate sizes quite independently of daylight.