Photography as practiced to-day is in a high state of perfection. Reliable materials and reliable chemicals are within the reach of every photographer who is a discriminating buyer, and through the use of these reliable materials uncertainty is practically eliminated.

Occasionally, however, trouble of a peculiar nature may arise, and a product that has been used successfully for years will apparently decide to cause trouble which is obscure and hard to locate.

A plate demonstrator in making his rounds called on a customer who was having trouble with fogged spots about the size of a dime near the center of plates while others were perfect. He had experienced this trouble off and on for some time and had saved about a dozen of the plates to show the demonstrator upon his arrival.

Here was a puzzler. Plates good in every way, with the exception of this diffused black spot in the center. The similarity of the marking in each instance convinced the demonstrator that the fault lay in the camera or in some local condition of handling. This the photographer did not deny, but said, "I have searched the camera thoroughly for light leaks and can't find anything wrong. Look it over yourself and see if you can."

The demonstrator inspected the camera carefully and like the photographer failed to find a leak of any kind. He and the photographer then went into the dark room, loaded the holders, and the demonstrator could see nothing wrong in the way the plates were handled. The slides were examined and they proved to be light tight. Some plates were exposed and a record of which plates were used in each holder was kept. The plates were developed and the trouble appeared on just one plate. This seemingly pinned the trouble down to the particular plate holder in which this plate was exposed, and noting the general location of the black spot on the negative, a further inspection of this holder was made and upon comparison to the other holders the demonstrator noticed that the spring clip in the back of the holder was rubbed and shining brass, while the other springs in the remaining holders were dull and blackened, and here was the cause of the trouble. The light shining through the lens and through the plate was reflected by this shiny brass spring clip and caused a diffused fog on nearly every plate - especially strong if in or near a high light in the negative. Here was trouble easily corrected but hard to locate. In most cases of trouble the condition is local as it was in this case, and in some it is fully as hard to trace to its source.

The proper way to trace trouble is by the method of elimination as this demonstrator did. He examined the camera for light leaks and found none. That eliminated light leaks in the camera as a possible cause and this method was used until all plate holders were free from suspicion except the one finally proved guilty.

This same method of elimination will work successfully in a large majority of cases and is applicable to every case regardless of the nature of the trouble or whether plates or paper are involved.

There is a cause for every effect and this cause is sometimes so elusive as to escape detection entirely. In the majority of cases, however, it is possible to find the local condition to which the trouble is due, and when the source of the trouble is located it is easy to correct it by removing the cause.

Most local troubles can be located and corrected by a thorough and systematic investigation and such a course is advisable for two reasons. First, because it is the quickest way to overcome a difficulty, and secondly, because the trouble thus located is a trouble that should it occur again is instantly recognized and the cause being known the remedy is easily applied. Be an investigator.