This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
We will take it for granted that our outfit consists of a Premo, Sr., 4 x 5 camera and three plate holders, for glass plates. As we said before, this camera represents a class and is very similar to many others on the market and we select it simply as a type of popular camera. The plate holders furnished with this camera have black rubber slides. We withdraw the slide and with a camel's hair brush we proceed to thoroughly dust, not only the holder but the slide. This is very important and should be done every time the holders are loaded. We also thoroughly dust the plate before insertion, not only on the film but on the glass side as well but do not overdo it or you will eloctrify the plate and attract the dust particles in the atmosphere.* The smallest speck of dust which may rest on your plate at the time of exposing it will act as a shield for that portion of the plate and thus cut the light off from it and when you come to develop and fix it, it will leave a small white hole in the plate. Now let us examine the holder and at the bottom we will find a small bar of wood. Press on the bar with your thumb nail and you find it recedes but again springs back into position. To load this holder, take it in the left hand and the plate in the right. Place the lower end of the plate in the groove in the spring bar at the bottom, keeping the film side of plate out, as shown in Fig. 9 and by pressing on the spring bar with the thumb the plate will fall into place. Keep the holder tilted back so that the plate will fall into the holder. To unload, press with the thumb on the spring bar, as shown in Fig 10, then by tilting forward the holder, the plate will fall forward, the edge resting against the fingers. The plate is then held in position by the ends only. When the plate is inserted replace the slide with the light side outward, or, if marked, see that the word "Exposed" is on the inside next to the plate. Close your eyes and go through the process of dusting your plate holders and plates and inserting in the holders. In this way you become famiiliar with the operation and when necessary you can change your plates in a dark closet without the aid of a ruby lamp. When you return the slide into the holder you will notice that it has to go into a little slit in the holder. This slit is closed automatically by means of a spring, so that when the plate is in the camera and the slide is withdrawn, no light can leak in upon the plate. In returning the slide do not insert one corner first but learn to insert the entire edge of the slide and to rapidly push it in to its proper place. If you insert one corner of the slide you open the slit and allow the light to enter but if you insert the entire edge the spring clamps around both sides of the slide and excludes the light. Of course it makes no difference in the dark room whether you insert the the corner or the entire edge of the slide but it does make a great difference when you are in the daylight and if you acquire a bad habit in the dark room you are very liable to do the same thing in the daylight and thereby ruin an exposure. The light creeping in at this small opening spreads over the plate and blackens it gradually from the opening inward. This blackening is technically known as "fog."
*See pages 107 and 108 in regard to the use of the air bulb instead of a brush.
A Portrait. Alfred Cox, Chicago.
Now it is well to number your plate holders from 1 to 6, i. e., two numbers to each holder, one on each side. You can do this with pencil or pen and ink but it will look neater to use a printed number, which may be gummed to the wood side of the holder just above the rubber slide. In this way you can keep track of your plates, as you expose them and make corresponding entries in your "Exposure Record." You will perhaps see no necessity for an exposure record but it is really a valuable if not an indispensable thing, if you expect to make good pictures and profit by your own experience. These books are for sale by dealers generally, or you can purchase a small memorandum book and rule and letter it yourself as shown in Fig. 11.
Five entries of this size could be made on each page of such a book and these entries will be found of great value when you come to develop your plate and for general reference at any time when you are about to take pictures under similar existing conditions. It will be noted that in the first column we have written 26 Seed. This shows the number and maker of plate. The next column shows that it was taken at 10:30 a. m., with diaphragm or stop 8 and the time of exposure was 1/50 of a second. The data further tell you that it was the 9th of August and plate holder was No. 3. The subject was a yacht just entering the harbor and it was consequently a sea and sky picture which, according to our lesson on light, we know to require but a very short exposure. The remarks tell us that although the exposure was a very short one, the picture would have been improved if we had given it but half the time or 1/100 of a second. All of these entries were made in the exposure record after the plate was exposed except the "26 Seed" and the "3" under holder number. These entries should be made as soon as the plates have been placed in the holders and then we will know just what our holders are loaded with in case they should be set aside for a few days or a week before using.