162 The Microscope. 



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 Eng- 

 land, the owner of a Powell and Leland 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 Messrs. Powell and Leland to be faultless. I 

 called on those gentlemen. We examined the objective to- 

 gether, and discovered on one of the combinations a film of 

 some substance which could not be removed except with alco- 

 hol. In five minutes 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 as 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 out- 

 side 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 T 4 ¥ ths-inch lenses to that treatment. I asked, " What are 

 you doing with that objective " " Putting it in order, at the re- 

 quest 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 re- 

 move the film of dust with which the exposed surface of the 

 back combination of an objective is sometimes 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 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 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 

 alone, when the parts have been separated, can replace them in 

 their original adjustment to the optical centre. Any other per- 

 son will be likely to screw in the cells either too tightly or not 

 tightly enough, and will thus throw the combinations out of 



