Spots.—One principal cause of spots is dust. The operating room should be kept as free from this as possible, and instead of its being dusted, it should be wiped with a damp cloth. Specks or flakes of iodide of silver are often found in the nitrate bath. These sometimes occur by an ever-iodized collodion, and sometimes by collodion falling off while being silvered. When this occurs, the nitrate of silver solution should be filtered. A new sponge or a tuft of cotton is a good article to filter nitrate of silver solution through. A small particle of light finding its way upon the plate, will produce a spot. Another and very frequent cause is, putting the slide of the tablet down rapidly, causing it to spatter upon the plate the solution which has drained off from it. This paper will be opaque when viewed by reflected light, and dark when viewed by transmitted light. Occasionally a sort of transparent spot will appear: this may be traced to a want of sensibility of the iodide of silver. Large transparent spots frequently appear by the operator's pouring the developing solution upon one place, and washing off the small quantity of nitrate of silver necessary to develope the image. This will be easily detected, and can be obviated by flooding the most of the surface of the glass with a steady stream of the developer.
Stains and Lines.—If the glass be allowed to rest for an instant with one portion of its surface in the silvering solution and the other out of it, it would cause a streak across; hence the necessity of totally immersing it with one firm, steady motion removing the glass before it has been thoroughly wetted, and the ether and alcohol allowed a uniform action over the entire surface. A plate should not be disturbed in the bath until it has been in a full minute at least.
Irregular Lines are often caused by using the developing solution too strong, or by not pouring it evenly over the plate at once. Should it be allowed to rest in its progress, if but for an instant, it will leave its line. Sometimes spangles of metallic silver appear: these are caused by the presence of too much nitric acid in the developer for the proportion of iodide in the film and the strength of the bath.
There are other phases connected with the practice of the positive process, which it would be almost impossible to commit to paper, and cannot be so explained as to be perfectly comprehended by the new experimenter. It is absolutely necessary for all to observe every little point noticed in the foregoing pages, and at the same time exercise some good judgment, and no one need hesitate through fear of not being successful.
OF THE
NEGATIVE PROCESS.