THE MEASUREMENT OF MICROSCOPIC OBJECTS. 293 



screw-threads by experimenting with an object which occupies 

 in the field of view the same space as that to be measured. If, 

 for instance, it is required to determine accurately the size of an 

 object whose image covers five divisions in the eye-piece micro- 

 meter, we select for the determination of the micrometer-divisions 

 or screw-threads a distance upon the stage-micrometer which 

 corresponds to the same number of divisions in the eye-piece. 

 Hence it also follows that the object to be measured must always 

 be brought into the middle of the field of view. 



The eye-piece screw-micrometer of Mohl cannot, therefore, afford 

 any other advantages than those which are in all cases combined 

 with the greater distinctness of the image in the middle of the 

 field of view ; and it is not to be supposed that these advantages 

 are capable of demonstration in objects of at most 20-30 mic. 

 in diameter, for which the instrument is intended. 



That very accurate measurements can be made with well- 

 constructed screw-micrometers is beyond doubt. According to 

 Harting the probable error of a single measurement amounts to 

 about J mic. only in one of Ploessl's instruments, and in the middle 

 of a series of measurements would be from ^ to T V mic. only. 

 In this case it is, of course, assumed that the observer is accurate 

 in the adjustments. If interference lines are mistaken by the 

 observer for the outlines of the object, or if he is confused by 

 optical effects of any other kind, he will obtain erroneous results 

 even with the most accurate instrument, especially if the object 

 to be measured is very small. Perfect vision in the observer, as 

 already stated, is in all cases the essential condition. 



Finally, as regards the means of denoting micrometric dimen- 

 sions, we think that Harting's proposal to use the micromillimetre 

 (= -001 mm.) as the standard of unity deserves general acceptance. 

 It would be advantageous to decide once for all upon a standard 

 of unity smaller than most of the objects which have to be 

 measured, because we are accustomed in other matters also to 

 measure both time and space by such unities. As the French 

 scale is undoubtedly the most generally employed in science, and 

 the micromillimetre forms, moreover, a convenient unit even for 

 the smallest objects which have to be measured, we do not think 

 there is anything valid to be said against Harting's suggestion. 

 Larger dimensions are always expressed in millimetres, and still 

 larger in metres, so that the determining number never exceeds 



