The Microscope. 163 



their necessary delicate relation to one another. Besides, un- 

 less skill and care be exercised in screwing the parts together, 

 the front and the middle combinations will sometimes be 

 brought in contact, and the flint glass, which is very thin at the 

 centre, will be broken. The screw-thread of the cells is very 

 delicate. Yet some persons, after failing to catch it, apply force 

 enough to break it. Such carelessness passes comprehension. 



A large-angle oil-immersion lens gets out of order easily. 

 If you find the definition of such objective to have lost its 

 sharpness, you may know that the front lens is out of centre. It 

 has come in contact with the slide. A very slight pressure is 

 sufficient to work the mischief. This susceptibility to injury is 

 unavoidable, as every optician will tell you. It is incident to 

 the requirements of high-angle construction. 



A few days ago an objective was sent to me with the re- 

 quest that the front lens should be reset. It had in some way 

 been forced out of its place. I reset it as well as I could. But 

 that objective, even if it had been repaired by its makers, the 

 Messrs. Powell and Leland, can never be what it was before the 

 injury. The only way of repairing it was by inserting a ring of 

 cement which, projecting slightly through the shoulder, neces- 

 sarily cut down the angle. A heavy shoulder means, of course, 

 a low angular aperture. 



A novel method of using an immersion lens came under my 

 notice recently. A water-immersion objective had been ordered. 

 It was made and sent, but it did not give satisfaction. I in- 

 quired by letter, " In what way do you proceed to work with it ?" 

 " I fill it with distilled water, and then screw it to the instru- 

 ment," was the reply. 



An objective is sometimes almost ruined through sheer 

 carelessness. I made a costly lens for a New York optician. 

 He tossed it several times in his hand, and finally dropped it 

 upon the floor. " Oh," he said, " that will not harm it ! " I 

 looked at it, and found the front combination tilted at an angle 

 of about forty-five degrees. This act of carelessness cost that 

 optician twent} r -five dollars. 



I have here the back setting of a 1^-inch lens which was 

 made by me several years ago. The purchaser of the lens had 

 screwed it so tightly to his microscope that he could not, with 



