The principal difficulty in the production of Negatives is to ascertain the right time of exposure to light and the proper point to which to carry the development of the image. A minor amount of fogging, stains, etc., is of less consequence, and will scarcely be noticed in the printing. With direct Positives however the case is different. The beauty of these pictures depends entirely upon their being clean and brilliant, without fogging, specks, or imperfections of any kind. On the other hand, the exposure and development of Positives is comparatively simple and easily ascertained. 1. _The shadows dark and heavy._--The plate has not received sufficient exposure in the Camera;--or the film being very transparent and the Silver solution weak, Nitric Acid is present in the Bath, or the Collodion is brown from free Iodine; in the latter case make the Collodion a little thicker, and develope with Sulphate of Iron in preference to Pyrogallic Acid. 2. _The shadows good, but the lights overdone._--The developing fluid may have been kept on too long; or the object is not properly illuminated (p. 220); or the Collodion is not adapted for Positives. 3. _The high lights pale and flat, the shadows misty._--The plate is over-exposed. Indistinctness of outline caused by over-exposure is distinguished from that produced by fogging by holding the plate up to the light; in the former case the image shows as a Negative. If the Collodion is colourless, clearer shadows will probably be obtained by dropping in Tincture of Iodine until a yellow colour is produced. 4. _The picture developes slowly; spangles of metallic Silver are formed._--Too much Nitric Acid is present in proportion to the strength of the Bath, to the amount of Iodide in the film, and to the quantity of Protosalt of Iron in the developer (p. 112). 5. _Circular spots of a black colour after hacking up with the varnish._--These are often caused by lifting the plate too quickly out of the Bath; or by pouring on the developer at one spot, so as to wash away the Nitrate of Silver; or by the use of glasses imperfectly cleaned. 6. _The image becomes metallic on drying._--If Sulphate of Iron is employed, the solution is too weak, or free Nitric Acid has been added in excess. If Pyrogallic Acid is used to develope, the proportion of Nitric Acid is too great. 7. _A green or blue tint in certain parts of the image._--This is caused by the deposit of Silver being too scanty, which may happen from over-action of the light, or from the film of Pyroxyline being _very thin_;--if the Collodion is diluted down beyond a certain point, the same quantity of free Nitrate of Silver is not retained upon the surface of the film. Add a few drops of the Bath to the developer before pouring it on the plate.