882 



EECORD OF CURRENT RESEARCHES RELATINa TO 



Fig. 97. 



coi:rse, much greater precision. The divisions at A and C, as also 

 tlie scale at G, give most exact and close readings for fixing the posi- 

 tion of a given jjoint of the object, and the stage has the advantage 

 that the position of the point is referred not to a curved line merely, 

 but to a particular portion of the segment of a circle which lies within 

 the dimensions of one field. 



When this stage is used as an object-micrometer, it must of course 

 (as with every screw micrometer), be used in the direction in which 

 the screw C works. It is well only to commence measuring after 

 the screw has been turned a little in the direction in which tbe 

 measurement is being made, as only by this means can the dead-way 

 be obviated, which is unavoidable when a screw turns backwards and 

 forwards. 



The measurements are read off directly from the divisions at A 

 and C. At A we have a value of ■ 25 mm., at C of 2 • 5 ^, and at the 

 nonius at C (to read which a lens is required), a value of ' 25 /x. 



Lastly, it should be mentioned that an objection which may 

 rightly be urged against the first stage, and which precludes its 

 use for exact scientific investigations, is got rid off entirely in 



the second form. It is this : the 

 turning of the upper movable 

 plate about the point D in the 

 first stage produces somewhat 

 excentric circles ; in the second 

 this is not the case, as the circles 

 formed about the point D are 

 exactly concentric." 



What are the mechanical 

 stages of English construction 

 which have found their way to 

 Germany, and to which the 

 above two stages are so greatly 

 superior ? 



"Fine" Adjustments.— The 

 crimes that are said to have been 

 committed in the name of liberty 

 are, we think, pretty well matched 

 by those committed in the name 

 of cheapness, at any rate when 

 perpetrated in the case of such 

 an instrument as the Microscope. 

 Fig. 97 shows a method actually 

 adopted in practice in a German 

 instrument for making the fine 

 adjustment. When the screw m is 

 withdrawn, the spx'ing seen above 

 the mirror presses the arm attached 

 to the stage against the pillar 



of the Microscope, and the stage then takes an oblique position. 



When the screw m is turned, it forces the arm outwards, and thus 



