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MCINTOSH BATTERY AND OPTICAL CO., CHICAGO. ILL., U. S. A. 55 part of the handkerchief with alcohol, and with the help, if needed, of the stick of wood in searching the corners, carefully clean each combination, and I then screw each cell back accurately to its place. The work is now finished, and I will attach the objective again to the microscope, and will again ask you to view the slide of diatoms through it. The dimness is now, you perceive, all gone. Indeed, you can hardly believe it the same objective, and you have ocular proof that cleanliness is essential to the best performance of a lens, and are witnessing an instance of the dependence of important results on attention to little things. “ Several years ago, while I was getting ready to visit England, the owner of a Powell and Lealand objective wished me to take the lens to its makers for correction or exchange. ‘ It is a poor lens,’ he said. I could not credit his statement, for I knew the work of the Messrs. Powell and Lealand to be faultless. I called on those gentlemen. We examined the objective together, and discovered on one of the combinations a film of some substance which could not be removed except with alcohol. In five minutes the lens was clean and in per- fect order; and to this day the owner refuses to believe that the lens which I brought back to him is the same that I took abroad. Never trust the cleaning of your objectives to the brass worker, or to any per- son who does not know how carefully a lens ought to be handled. The brass worker will polish the outside of the objective, but will get the lenses out of center. To my great disgust, I once found a brass worker subjecting one of my 4-10 inch lenses to that treatment. I asked, ‘ What are you doing with that objective ? ’ ‘ Putting it in order at the request of the owner,’ he said; ‘he wants to sell it.’ Taking the lens, I cleaned it for him without charge. A camel’s hair brush can neither completely nor safely remove the film of dust with which the exposed surface of the back combination of an objective i? sometimes found to be coated. It will make a senes of rings on the surface of the lens, and it may, if grit be present, scratch the glass. Nor should the handkerchief, either wet or dry, be introduced into the tube of any but a low power objective. The cells must first be unscrewed from their mountings, and then the cleaning be done properly. But, let me add, an objective ought never to be taken apart by any one but its maker. He has the lathe upon which it was made, and he alone, when the parts have been separated, can replace them in their original adjustment to the optical center. Any other person will be likely to screw in the cells either too tightly or not tightly enough, and will thus throw the combinations out of their necessary delicate relation to one another. Besides, unless skill and care be exercised in screwing the parts together, the front and the middle