Part 7

3. Never use damp cloths, leathers, or buffs, for giving the final polish to the plate. Negatives with an indistinct and muddy surface are frequently produced from this cause.

4. Let the film set properly before immersion in the nitrate of silver bath: its condition can be ascertained by gently touching the lower part of the coated plate with the end of the finger.

5. Never omit to pass a broad camel-hair brush over the plate just before pouring on the collodion.

6. Bear in mind that, as light is the producing agent, so will it prove a destructive one: not less than four folds of yellow calico should be used to obstruct white light; and in that case the aperture covered should be no larger than is necessary to admit sufficient light for working by. Examine occasionally the yellow calico: when this material is used to exclude white light, it becomes bleached by constant exposure. Do not trust alone to any colored glass; no glass yet made is anti-actinic under all aspects of light and conditions of exposure.

7. When the negative requires intensifying, carefully wash off all traces of the first developing solution before proceeding to intensify. This operation may be performed either before or after the iodide is removed by fixing.

8. Glass baths are preferable to porcelain, ebonite, or gutta-percha baths for solution of nitrate of silver.

9. In using either spirit or amber varnish, before pouring it off, keep the plate horizontal a few seconds. This gives time for soaking in, and prevents the formation of a dull surface arising from too thin a coating.

10. Rub the lenses occasionally with a soft and clean wash-leather, the rapidity of action is much influenced by the brightness of the lenses: their surfaces are constantly affected by moisture in the atmosphere, which condensing, destroys the brilliancy of the image.

11. The white blotting-paper used for some photographic purposes is not suitable for filtering solutions; that only should be employed which is made for this purpose, and is sold under the name of filtering-paper.

12. Hyposulphate of soda.--A great deal of rubbish is sold under the name of this salt. As a test of its quality, 1-1/2 drachms should entirely dissolve in 1 drachm of water, and this solution should dissolve rather more than 4-1/2 grains of iodide of silver.

13. Chemicals.--The purity of photographic chemicals cannot be too strongly urged; the cheapest are not always the most economical. The commercial preparations are generally not to be depended upon, as these, though perhaps unadulterated, are, strictly speaking, not chemically pure. It is best to procure them from well-known chemists, who understand the purpose for which they are intended, and make the preparation of these substances peculiarly a branch of their business.

14. Never leave chemical solutions exposed in dishes: when done with, pour them back into glass-stoppered bottles, and decant for use from any deposit, or filter if necessary.

15. In all photographic processes it is absolutely necessary to be chemically clean; and this sometimes is not easy. As a rule, never be satisfied with cleanly appearances only, but take such measures as shall insure the absence of all extraneous matter in preparing the solutions, cleaning the glasses, dishes, etc.

16. All stains on the hands, linen, etc., may be removed by means of cyanogen soap or cyanide of potassium, which should be applied without water at first, then thoroughly washed off. To assist the operation, the hands may be now gently rubbed with a fine piece of pumice-stone, when the stains quickly disappear.

For more perfect and complete directions, the reader is referred to any complete work on photography.

MECHANICS.

There is no subject of such importance as Mechanics, as its principles are founded upon the properties of matter and the laws of motion; and in knowing something of these, the tyro will lay the foundation of all substantial knowledge.

The properties of matter are the following: Solidity (or Impenetrability), Divisibility, Mobility, Elasticity, Brittleness, Malleability, Ductility, and Tenacity.

The laws of motion are as follows:--

1. Every body continues in a state of rest or of uniform rectilineal motion, unless affected by some extraneous force.

2. The change of motion is always proportionate to the impelling force.

3. Action and reaction are always equal and contrary.

EXPERIMENT OF THE LAW OF MOTION.

In shooting at “taw,” if the marble be struck “plump,” as it is called, it moves forward exactly in the same line of direction; but if struck sideways, it will move in an oblique direction, and its course will be in a line situated between the direction of its former motion and that of the force impressed. This is called the resolution of forces.

BALANCING.

The center of gravity in a body is that part about which all the other parts equally balance each other. In balancing a stick upon the finger, or upon the chin, it is necessary only to keep the chin or finger exactly under the point which is called the center of gravity.

THE PRANCING HORSE.

Cut out the figure of a horse, and having fixed a curved iron wire to the under part of its body, place a small ball of lead upon it. Place the hind legs of the horse on the table, and it will rock to and fro. If the ball be removed, the horse would immediately tumble, because unsupported, the center of gravity being in the front of the prop; but upon the ball being replaced, the center of gravity immediately changes as position, and is brought under the prop, and the horse is again in equilibrio.