652 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



may be determined by its distance from two fixed points or lines on the 

 same plane. The slide is placed in a frame A and kept always in 

 the same position by a rod X. Before the slide is inserted the rod X is 

 pressed forward to the anterior margin of the frame where it is held by 

 two teeth M. By pressing the knobs of the teeth, the rod is released 

 and springs back so as to fix the slide in a given position. The frame is 

 moved from right to left by a micrometer screw C. On an immovable 

 plate K, a permanent point o is marked, and the adjacent margin of the 

 movable frame is divided in 0*25 mm. Thus one line on the preparation 

 is defined. But the frame A is fixed to another movable plate BB 

 which is worked by the rack and pinion G, D, on an inferior fixed plate 

 E E, and in an antero-posterior direction. One margin F of this fixed plate 

 is also graduated, and there is another fixed point o, so that the desired 

 point in the field can be defined in two directions, and therefore readily 

 determined. The plates B B and E E have apertures for illumination. 

 The attachment to the stage is a simple matter. 



The whole field can be systematically observed, a point can be registered 

 and readily found again, the size of large objects can be measured, 

 movements of organisms can be defined, and the comparison of lent 

 preparations greatly facilitated. 



Notwithstanding the fulness of the description and the renown of the 

 German periodical in which it appears, it must be said that the " Microstat " 

 is simply a mechanical stage with finders, and in this country at any rate 

 bas no feature of novelty. 



Darling's Screw-Micrometer.* — Mr. S. Darling has devised two forms 

 of screw micrometer, in which he claims there is " no perceptible play 

 between the threads of the screw and the nut," and in which " the screw 

 will revolve much farther, relative to the motion of the cross-hairs, than in 

 the micrometers heretofore made ; " and further, that he has found " a sub- 

 stitute for the common cross-hairs (spider's web), by whicb measurements 

 can be made with greater accuracy and uniformity." 



One form of his micrometer (fig. 174, top view, with top E removed ; 

 fig. 175, section of fig. 174 through A B) has a V-thread screw and nut, the nut 



Fig. 174. 



being split at one end and a screw tightening the nut. The frame that 

 carries the cross-hairs has a very small hard abutting-piece coming against 



* Specification of U.S. Patent, No. 287,420, Oct. 30, 1883. 



