Secondly, to _whiten_ the surface of the reduced metal as much as possible, in order to produce a sufficient contrast of light and shade. Iodide of Silver developed in the usual way presents a dull yellow appearance which is sombre and unpleasing. _The Collodion and Nitrate Bath for Positives._--Good Positives may be obtained by diluting down a sample of Collodion with Ether and Alcohol until it gives a pale bluish film in the Bath. The proportion of Iodide of Silver being in that case small, the action of the high lights is less violent, and the shadows are allowed more time to impress themselves. The dilution lessens the amount of Pyroxyline in the Collodion at the same time with the Iodide, which is an advantage, the slight and transparent films always giving more sharpness and definition in the picture. The employment of a very thin film for Positives is not however always a successful process. The particles of the Iodide of Silver being closely in contact with the glass, unusual care is required in cleaning the plates in order to avoid stains; and the amount of free Nitrate of Silver retained upon the surface of the film being small, circular patches of imperfect development are liable to occur, unless the reducing agent be scattered evenly and perfectly over the surface. Also if free Iodine or organic substances which have a retarding effect on the action of light are present to a considerable extent, the Collodion will not work well with a small proportion of Iodide. The Author found in experimenting on this subject that with perfectly pure Collodion and a _neutral_ Bath most vigorous impressions were produced when the density of the film had been so far reduced by dilution that scarcely anything could be seen upon the glass; but with Collodion strongly tinted with Iodine, or with a Bath containing Nitric Acid, it was necessary to stop the dilution at a certain point or the film became absolutely insensitive to feeble radiations of light, and the shadows could not be brought out by any amount of exposure. In this case, by adding more Iodide a better effect was obtained. A thicker Collodion may be used for positives if a little free Iodine be added, for the purpose of diminishing intensity and keeping the shadows clear during the development. This process is easier to practice than the last, but does not always give the same perfect definition. No organic substance of the class to which Glycyrrhizine and Sugars belong should be added to Collodion which is to be used for Positives. By so doing the image would be rendered intense, and the high lights liable to solarization, _id est_, a dark appearance by reflected light.