114 JOURNAL OF THE [May 



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 min- 

 utes the lens was clean and in perfect order ; and to this day the 

 owner refuses to believe that the lens which I brought back to 

 him is the same with that which I took abroad. 



Never trust the cleaning of your objectives to the brass-worker, 

 or to any person 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 centre. To my great 

 disgust, I once found a brass-worker subjecting one of my Aths- 

 inch lenses to that treatment. I asked, " What are you doing 

 with that objective?" "Putting it in order, at the request of 

 its 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 com- 

 bination of an objective is sometimes found to be coated. It 

 will make a series of rings on the surface of the lens, and it may, 

 if grit be present, scratch the glass. Nor should the handker- 

 chief, 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 can 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 



