_The Background_ best adapted to positives is unbleached muslin, such, as is sold for sheeting, and can be found in almost any dry goods' store: it should be from two to three yards wide. A clouded appearance is given to the background by merely marking it with _charcoal_, forming streaks or "_waves_" resembling clouds. These come out black, or dark, in the positive, and give a variegated appearance. The roughness of the marking does not matter, since the background is generally a little out of the focus of the lenses. Trees and other designs may be represented in this matter. _Positive Collodion Pictures_ may be whitened by the use of bichloride of mercury, thirty grains to one ounce of water. After the picture has been developed, fixed and washed, by the process given in the preceding pages, the solution of bichloride of mercury may be poured over the surface of the image: it almost immediately presents a series of interesting changes in color. The surface at first blackens but for a few minutes, when it begins to get lighter, and gradually brightens to a pure white, inclining to a blue. It should then be thoroughly washed and dried, as usual. _Instantaneous Pictures_ may be taken by employing the _iodide of iron_ in the collodion. The best method with which I am acquainted, is to have a saturated solution of iodide of iron in alcohol, and drop one or two drops into an ounce of the collodion (which has been previously iodized) used in taking positives. This can be used at once, as soon as mixed. No more of this collodion should be prepared than is wanted for immediate use, as it will keep good for only a few hours. The saturated solution of iodide of iron can be kept on hand ready for use at all times. There should be an excess of the iodide in the alcohol. The same accelerating effect is produced with the negative collodion prepared in this way. _Plain Collodion_ may be iodized as soon as dissolved: this will save time in settling. It is a fact that some cotton is more readily dissolved when the bromo-iodizing is present: but by the addition of this, I have often taken up considerable quantities of the gummy sediment remaining in the bottom of bottles. _Prepared Glass_ may be preserved clean and free from dust by keeping in boxes such as those used for keeping daguerreotype plates. By taking out every other partition between the grooves, the glass can be readily put in. _Collodion Vials and Bottles_, after having collodion in them once, should be rinsed with alcohol and ether (in the same proportions as employed for dissolving the cotton), before using them a second time for that purpose. Penuriousness in this respect would be bad policy. _Coating Large Glasses._--A convenient method of doing this is represented in the following illustration: [Illustration] The glass is held by one corner, 2; the other corner, 4, is rested on a table or bench, and the collodion poured on, so that the excess may be poured off at 1.