014 SUMMARY OF CURRENT RESEARCHES RELATmO TO 



not necessary to take tlic mean of the two Microscope readings einco 

 their sum will give the mean directly. A glance into tbc IMicroscopes 

 is sufficient to give the mean of the readings and scarcely occnpies a 

 quarter of the time ncccs-ary for vernier readings. The scale gradua- 

 tions, which cannot be made so fine upon metal as the micrometer 

 graduations upon glass, and which are magnified three to five times by 

 the objective, appear much broader than the latter. With ordinary 

 instruments which arc finely divided a line on the scale covers at least 



Fig. 101. 



30 seconds to one minute, and from this fact would result a source of 

 error if means were not found to obviate it. Instead of using the whole 

 breadth of the mark the attention is confined to the same edge of it, 

 namely that which is on the right-hand side towards the long mark. It 

 is still better if the graduations terminate at one end in a point, such as 

 is generally produced by the graving tool ; but the pointed end should 

 always be that at which the divisions are level, and not towards the pro- 

 longations of the whole degrees and half degrees. The tenths, &c., can 

 then be very accurately estimated if the micrometer divisions project 

 beyond the pointed ends (fig. 101)." 



The divisions should be short (not more than 1/2 mm. in length), 

 and as fine as possible ; the exact coincidence of ten divisions in the 

 micrometer with one division of the scale is secured as nearly as possible 

 by preliminary calculations and then made absolute by a slight movement 

 of the objective-tube. 



With powerful Microscopes it is desirable to have some simple and 



