Then a final rinsing and drying is in order. Many operators use a final washing with alcohol of at least 90 per cent strength which is allowed to evaporate with little or no wiping. Alcohol denatured with methyl alcohol serves well if strong enough but beware denatured alcohol of unknown composition. Others have used petroleum naphtha and things of that sort. At the present time these commercial petroleum products are extremely uncertain in quality, like gasoline, being obtained, Heaven knows how, from the breaking down of heavier petroleum products. If pure petroleum ether can be obtained it answers quite as well as alcohol, but unless the volatile fluid is pure it may leave streaks. Ordinarily neither has to be used, as after the proper wiping the glass comes perfectly clean. This done the glass can be replaced on the pad whence it came and its mate put through the same process. Flint glass is more liable to spot than the crown, but the crown is by no means immune against the deterioration of the surface, perhaps incipient devitrification, and during the war many objectives “went blind” from unexplained action of this character. As a rule the soap and water treatment applied with care leaves even a pretty hard looking specimen of objective in fairly good condition except for the scratches which previous users have put upon it. Then if the spacing pieces, usually of tinfoil, are not torn or corroded they can be put back into place, the one lens superimposed upon the other, and the pair put back into the cell by dropping it gently over them and re-inverting the whole, taking care this time to have soft cloth or lens paper under the fingers. Then the retaining ring can be put into place again and the objective is ready for testing or use as the case may be. If the spacers are corroded or damaged it may be necessary to replace them with very thin tinfoil cut the same size and shape, leaving however a little extra length to turn down over the edge of the lower lens. They are fastened in place on the extreme edge only by the merest touch of mucilage, shellac or Canada balsam, whichever comes to hand. The one important thing is that the spacers should be entirely free of the sticky material where they lap over the edge of the lens to perform the separation. This lap is generally not over 1/16 of an inch, not enough to show at the outside of the objective when it is in its cell. When the upper lens is lightly pressed down into place, after the gum or shellac is dry, all the projecting portion can be trimmed away with a sharp pen-knife leaving simply the spacers in the appointed places from which the original ones were removed. Some little space has been given to this matter of cleaning objectives, as in many situations objectives accumulate dirt rather rapidly and it is highly desirable for the user to learn how to perform the simple but careful task of cleansing them.