146 Transactions of the Society. 



and 9oc>oo i n -) it vvas f° ulu 'l advisable to employ oblique light 

 from an oil-immersion condenser, and only ten lines of these two 

 latter bands were spanned, the fixed wire being brought to the 

 first ruling of each of these groups, and continuous readings of the 

 movable wire taken as it was screwed up to the edge of each line 

 in succession,* thus every line was spanned with a wholly different 

 portion of the screw, but, nevertheless, the additions of the 

 readings of the first eight lines of the eighth group, and the first 

 nine of the ninth, afford true iooo"o m - readings free from screw 

 error, being effected with the same portion of the screw used for 

 the yoooo' i Q - spaces of the first band. 



In conclusion, I should mention that it would have been 

 practically impossible to properly effect the above measurements 

 had the micrometer not been supported on a separate pillar-stand 

 designed by Mr. Nelson many years ago. In this way the micru- 

 meter is brought close to, but not actually touching ; the Microscope 

 tube, thus turning the drum, imparts absolutely no vibration or 

 movement to the most highly magnified image. The common 

 rough-and-ready method of fitting the micrometer into the tube of 

 the instrument itself in the manner of an ordinary eye-piece, is 

 fatal to accuracy. A micrometer, provided with a screw-traversing 

 frame, carrying both wires, as described, and figured in " Carpenter," 

 would have greatly facilitated the work and have saved much 

 labour. It speaks volumes, however, for the wonderful workman- 

 ship of the mechanical stage of Powell's large stand that the fixed 

 wire could be brought into accurate position by such means under 

 the magnification employed. A more crucial test for accuracy of 

 workmanship could hardly be devised by the most exacting micro- 

 scopist, and I venture to assert that few instruments would 

 satisfactorily withstand such an ordeal. 



* The measurements of the lines of bands 4,5, 6, and 7 were also obtained in this 

 way, the fixed wire being, however, adjusted to the edge of each line of the first 

 three groups. In measuring the five xvhns in. intervals in bands 2, 3, 4, 5, 6, and 

 7, the fixed wire was accurately adjusted each time. It was not found feasible to 

 adjust the fixed wire so as to obtain five ^t^rrr in. readings throughout bands 

 8 and 9. 



