This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Margins Or Corners Of Plate Either Transparent Or Semi-Transparent. Caused by not leaving the star-diaphragm in front of the lens for a sufficient length of time during the exposure. The star-diaphragm should remain in position for at least three-fourths of the time. The lighting conditions and the character of the subject regulate the amount of time to a certain extent. A series of practical demonstrations has shown that the star-diaphragm should remain in front of the lens during thirteen-six-teenths of the exposure. If, on developing the plate, you observe that the corners and edges have not received as much exposure as the center, carry the development beyond the normal for the central portion; in fact, develop until the margins of the image are of proper strength. After fixing the plate, take a weak reducing solution (Red Prussiate of Potash) and locally reduce the central portion of the plate until the whole image is of uniform density. This method of procedure is not to be encouraged, but when one has a negative of a valuable subject and this difficulty occurs, it is advisable to proceed as directed.
Opaque Circular Spots In Center Of Plate. Caused by over-exposing the center - not leaving the star-diaphragm in position long enough during the exposure to give uniform illumination. To save such a plate, after fixing it reduce the central opaque spot until its density matches that of the surrounding portion.
Opaque Star-Shaped Spot In Center Of Plate. Caused by the star-diaphragm not being revolved during the exposure. Remedy obvious.
Transparent Spots In The Corners Of The Sky Portion Of The Plate. Caused by the hand or finger coming within the angle of view of the lens while making the exposure. As the angle of view is so great, when removing the cap, the hands must be immediately drawn away from the front of the camera. If this precaution is not taken, there will be danger of cutting off rays of light and thus producing transparent spots on the sky portion of the plate.