104 Prof. Richard Threlfall on the 



state at once exactly how it was done. A thread having 

 been selected by preliminary trial for uniformity, was broken 

 to the required length. A sample about a quarter of an inch 

 long was then broken off each end and placed on a microscope 

 slide. The refractive index of quartz is about the same as 

 that of Canada balsam, which therefore had to be avoided; 

 consequently the ends were mounted in 50-per-cent. glycerine 

 stained with aniline blue, and a carefully chosen cover-slip 

 placed over all as in ordinary microscope practice. The object 

 of colouring the glycerine was to make the quartz more visible. 

 The objective used was a Zeiss D, giving a magnifying power 

 of about 230 diameters with the eyepiece used. The ordinary 

 camera was employed to give a simultaneous view of a scale 

 pinned to a stand on the table at such a height that parallax was 

 almost entirely avoided. The thread to be measured was then 

 brought to the centre of the field, and a reading of the scale 

 was taken. The process was repeated with the other end, 

 and finally the threads were replaced by the micrometer. It 

 was found necessary to use the same part of the field for the 

 micrometer that had been occupied by the thread in making the 

 comparison by the aid of the step-scale. A most laborious pre- 

 liminary examination was made in order to avoid constant 

 errors due to the micrometer. The Zeiss scales divided into 

 hundredths of a millimetre were compared with a very fine 

 scale by Powell and Lealand. The Zeiss scales were found to 

 agree, but to give readings differing from the Powell and 

 Lealand scale by about 3 per cent, in the direction of making 

 the divisions of the latter too large. The millimetres of each 

 of these scales (/. e. the end divisions) were then compared 

 directly by means of a very fine comparing-microscope by the 

 Cambridge Scientific Instrument Co. The screw of this 

 instrument had been minutely examined previously by Mr. 

 Pollock for another purpose, and had been found more correct 

 than it was possible to set the head (to *0001 inch) for a 

 certain short part of its run. The result of this comparison 

 again showed that the Powell and Lealand differed from 

 the Zeiss micrometers by from 3 to 4 per cent. The divisions 

 of all the micrometers were then stepped on the stage and 

 found to be all correct as nearly as could be estimated ; in 

 other words, they were identical in length, or did not differ 

 from each other by more than 3 per cent, of their whole 

 length. The sensitiveness is indicated by the observation that 

 at the centre of the field 3*76 division of the step-scale corre- 

 sponded to "01 on the micrometer. Finally, to decide which 

 micrometer was the best to use, the Cambridge Instrument 

 Co.'s comparing-microscope was used to compare the milli- 



