1885.] NEW-YORK MICROSCOPICAL SOCIETY. 115 



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 their 

 necessary delicate relation to one another. Besides, unless skill 

 and care be exercised in screwing the parts together, the front 

 and the middle combinations will sometimes be brought in con- 

 tact, 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 sharp- 

 ness, 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 request 

 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 Lealand, 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, necessarily 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 inquired 

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

 fill it with distilled water, and then screw it to the instrument," 

 was the reply. 



An objective is sometimes almost ruined through sheer care- 

 lessness. 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 carlessness cost that optician 

 twenty-five dollars. 



