2. _Turbidity of the Nitrate solution._--_a._ From flakes of Iodide of Silver having fallen away into the solution, by use of an over-iodized Collodion.--_b._ From a deposit formed by degrees upon the sides of the gutta-percha trough.--_c._ From the inside of the trough being dusty at the time of pouring in the solution. In order to obviate these inconveniences, it is well to make at least half as much again of the Nitrate solution as is necessary, and to keep it in a stock-bottle, from which the upper part may be poured off when required. The frequent filtration of Silver Baths is unadvisable, since the paper employed may be contaminated with impurities. 3. _Dust upon the surface of the glass at the time of pouring on the Collodion._--Perfectly clean glasses, if set aside for a few minutes, acquire small particles of dust; each plate should therefore be gently wiped with a silk handkerchief immediately before being used. 4. _Faults of the Slide._--Sometimes a small hole exists, which admits a pencil of light, and produces a spot, known by its being always in the same part of the plate; occasionally the door works too tightly so that small particles of wood, etc., are scraped off, and projected against the plate when it is raised. Or perhaps the operator, after the exposure is finished, shuts down the door with a jerk, and so causes a splash in the liquid which has drained down and accumulated in the groove below; this cause, although not a common one, may sometimes occur. 5. _Insoluble particles in the Pyrogallic Acid._--The solution of Pyrogallic Acid will not usually require filtering, but if specks of Metagallic Acid are present, the developer should be passed through blotting paper before use. Spots of Transparency may generally be traced to some cause _which renders the Iodide of Silver insensible to light at particular points_, so that on the application of the developer no reduction takes place. 1. _Concentration of the Nitrate of Silver on the surface of the film by evaporation._--When the film becomes too dry after removal from the Bath, the solvent power of the Nitrate increases so much that it eats away the Iodide and produces spots. 2. _Small particles of undissolved Iodide of Potassium in the Collodion._--These are likely to occur when Anhydrous Ether and Alcohol are employed. They produce transparent specks at every part of the plate. Allow the Collodion to settle, or add a drop of water, which will dissolve the Iodide. 3. _Alcohol or Ether containing too much water_.--This causes a reticulated appearance of the film, which is rotten and full of holes. 4. _Use of glasses improperly cleaned._--This cause is perhaps the most frequent of all, when the film of Pyroxyline is very thin and the Bath neutral. After glasses have been long used it is often difficult to clean them so thoroughly that the breath lies smoothly; but the use of Potash gives the best chance. MARKINGS OF VARIOUS KINDS ON COLLODION PLATES.