APPENDIX. 



335 



of M. Nachet having been long directed to this point, he has recently 

 brought out a form of < porte-objectif ' (an improvement on a suggestion 

 originally made by Prof. Thury) which allows the change of objectives to- 

 be readily made without as much raising of the body from the stage as is 

 required in screwing and unscrewing. It consists (Fig. 502) of a fixed 

 inner cylinder, whose top screws into the bottom of the body; this being 

 embraced by a movable outer 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 circumference, so as to allow a collar (B) at the top of 

 the objective to be slipped into the opening as shown at c. When this is 



FIG. 502. 



Nachet's Objective carrier. 



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 is supplied by M. Nachet (when desired) with the 

 Society's screw, and the 'collar' can be adapted to receive either M. 

 Nachet's or any other Objectives. Having been enabled, by the kindness 

 of M. Nachet, to make a trial of this little apparatus, the Author is glad 

 to be able to speak most favorably both of its simplicity and its effective- 

 ness. 



