662 



SUMMARY OF CUERENT RESEARCHES RELATING TO 



Fig. 146. 



cylinder A, that is kept closely pressed up to its lower end by a strong 

 spiral spring between the two. The bottom of this outer cylinder is 

 formed by a shoulder that is cut away for about one-fourth of its cir- 

 cumference, so as to allow a collar B, at the top of the objective to be 

 slipped into the ojieniug as shown at C (Fig. 145). When this is 

 done, the objective is held firmly in place by the pressure of the 

 spring ; and all that is needed to remove it is a slight pulling down 

 of the outer cylinder, which enables the collar of the objective to be 

 slipped out again. The inner cylinder can have the Society screw ; 

 and the " collar " can be adapted to receive either M. Nachet's or any 

 other objectives. 



Having been enabled to make a trial of this little apparatus, 

 Dr. Carpenter is " glad to be able to speak most favourably both of its 

 simplicity and its effectiveness." 



Verick's Objective " Extractor." — This (Tig. 146) is also a con- 

 trivance for readily attaching and removing the objective, and which 

 is in principle an adaptation of that of M. Nachet. The nose-piece 



(which screws into the body) has 

 an outer tube somewhat shorter, to 

 which is attached a semicircular 

 "fork" projecting at right angles 

 at the bottom. The outer tube 

 is ordinarily pressed back by an 

 internal spring, which brings the 

 fork against the lower end of the 

 nose-piece. On pressing the catch 

 G which forms part of the outer 

 tube, the latter descends, and the 

 fork is separated from the nose- 

 piece. The objective E, which has 

 a wide collar, can then be slipped 

 into the fork, and on releasing the 

 tube the fork, with the objective, 

 is drawn back by the spring. A 

 short cylindrical piece projecting 

 above the collar of the objective 

 fits into a corresponding aperture 

 at the end of the nose-piece, and 

 thus secures accuracy of centering. 

 Whilst this apparatus undoubtedly allows a change of objectives 

 to be made with the utmost rajiidity, we find that the objectives are 

 not retained in position with sufficient firmness, slight pressure being 

 sufficient to deflect them laterally ; this is especially noticeable in 

 using an objective with " correction " collar. 



Sliding Objectives. — A modified method of accomplishing this 

 object * is applied by Messrs. Parkes to a very cheap form of instru- 

 ment. The end of the body-tube terminates with a smaller sprung 



* Sec tliia Journal, iii. (1S80) p. 1048. 



