ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



117 



Rogers' Micrometers. — Prof. W. A. Eogers, of Cambridge, U.S.A., 

 recently offered, as we announced,* to present a ruled stage micro- 

 meter to any one who would undertake to examine its divisions and 

 publish the results. Mr. T. S. Bazley having accepted the proposal, 

 now details the result of the investigation.! " Placed on the stage, 

 and viewed with a two-thirds objective, and a dark field, the ruled 

 lines, which are not filled in with a dark pigment as is common, 

 sparkle like streaks of diamonds ; and under this illumination a 

 singular appearance is noticed. In some of the lines a slight internal 

 splintering of the glass has apparently followed the course of the 

 ruling-point, giving an effect of deeper cuts in certain places. But, 

 as this effect is invisible with a bright field, and as there is certainly 

 no variation in the width of the several lines, it probably arises solely 

 from the nature of the glass ; and the more so, as these apparently 

 deeper cuts do not often extend for the entire length of a line, and 

 sometimes occur side by side for a few lines. 



" The micrometer is of the ordinary 3 by 1 size. The ruled portion 

 is a centimetre in length, and contains 1000 spaces, subdivided at 

 every fifth and tenth, the lines being thus • 01 mm. apart. The width of 

 the band, neglecting those lines that project, is 1 • 375 mm. Every 

 tenth line is 1 • 6 mm. long, and the principal spaces of 6 • 1 mm. are 

 subdivided by a shorter pro- 

 longation of the fifth lines, Fig. 24. 

 which measure 1 • 55 mm. 

 These measurements are the 

 average only, for the lengths 

 of the individual lines vary a 

 few thousandths of a milli- 

 metre, and the lower edge of 

 the band is not consequently 

 strictly in one straight line. 

 The terminations of the lines 

 at the upper edge, indepen- 

 dently of those projecting at 

 every fifth and tenth, are not 

 in the same straight line 

 either. These deviate in a 

 symmetrical manner ; four 

 lines between two long ones 

 having their ends equal and straight, while the ends of the next four 

 form a gentle convex curve. All the lines at this, which may be con- 

 sidered the reading edge of the band, are terminated by singular 

 hooks, suggestive of the curved handle of a walking-stick (see Fig. 

 24) ; they differ somewhat in size and character, but have all the 

 same direction, and are probably due to the stopping, lifting, and 

 reversal, of the cutting diamond. 



" The objectives used were a series by several makers (dry, as well 

 as immersion adapted to various media) up to Zeiss's L, equivalent to 



* See this Joiu'nal, i. (1881) p. 678. 



t Engl. Mech., xxxiv. (1881) pp. 341-2 (1 %.)• 



