858 SUMMARY OF CURRENT RESEARCHES RELATING TO 



more be endured, since on the one hand each of such objectives can be 

 adjusted according to desire for the short Continental or for the long 

 English tube, and thus effect can be given to personal inclinations ; 

 while on the other hand, where it is a question of sharpest observa- 

 tion, it is easy to provide a suitable medium thickness of cover-glass 

 where the immersion fluid is not exactly uniform with crown-glass. 

 Under all circumstances one gives up in using the fixed mounting 

 only unessential conveniences and benefits hardly worth consideration, 

 whilst far greater advantages are gained and very considerable defects 

 avoided. 



In conclusion, therefore, Dr. Dippel repeats : — " For all histolo- 

 gical and similar scientific observations, hold firmly to the fixed 

 mounting for homogeneous-immersion objectives. And if we have 

 such an objective with correction-collar, I say with Prof. Abbe, ' after 

 careful testing of the best correction for medium conditions by means 

 of the silver plate, screw it up tightly (" niet und nagelfest," clinched 

 and riveted), so that no mischief can arise.' " * * 



Nelson's Adapter for Rapidly Changing Objectives.— This ap- 

 pliance has been devised by Mr. E. M. Nelson to facilitate the 

 rapid interchange of objectives without the necessity of triple or 

 quadruple nose-pieces, or such an alteration of the existing system 

 Fig 159 as wou ^ prevent the free interchange of objectives 



L^J provided with the normal Society screw, as is the 

 N IpS W I case with the devices of Partes, Nachet, and 



(KL^ygjIlJft In Fig. 159 N is an adapter, - ]" the inner screw- 



thread of which is filed down smooth in three equal 

 Sand equidistant segments, leaving the thread intact 

 in the intervening three segments. The screw- 

 thread on the objective is filed down in three places 

 to correspond with N, so that where the gauge-slots 

 S and S' coincide the objective can be pushed in for 

 the length of the screw, and then an eighth of a 

 turn to the right screws it securely " home," just as 

 it would be after tbe four turns required with the 

 Society screw aud ordinary nose-piece. Similarly to detach it, only 

 an eighth of a turn to the left is necessary. Whilst the objective can 

 be inserted at any of the three positions in which the segments of 

 the nose-piece and objective coincide, it is only in the one position 

 where the gauge-slots S and S' coincide, that the screw-threads 

 correspond, and the one-eighth turn for screwing "home" can be 

 made without injury to the threads. J 



* Cf. the discussion on this paper, Proceedings, post. 



f By a mistake of the engraver the outer screw of N is drawn of less diameter 

 than that of 0. They are both of Society gauge. 



t Since the construction of the adapter, Mr. Nelson's attention has been 

 called to a communication in ' Science-Gossip,' 1879, p. 18, in which Mr. James 

 Vogan suggested a similar system under the heading " A Substitute for Nose- 

 pieces." The plan then proposed by Mr. Vogan involved cutting away two 

 segments of one-fourth the circumference of the screw-thread. 



