290 TECHNICAL MICROSCOPY. 



apparatus itself will be requisite in exceedingly rare instances 

 only. The errors which are due to the instrument are almost 

 always infinitely small in comparison with those which the 

 observer commits by inaccurate adjustment. In questions where 

 these errors of adjustment might be of importance, no microscopist 

 would be willing to compare the measurements of another with 

 his own ; he would, at most, take into account only the figures 

 of the same series of observations. It might with confidence be 

 affirmed that if measurements of different observers admit of 

 comparison in any investigation, this comparison still exists if 

 the respective quantities are taken 5-10 per cent, greater or 

 less. 



When observers state that they have refined the accuracy of 

 direct measurement up to 1 mic. by continued repetition of the 

 process, such assertions are to be regarded as deceptive, because 

 the nature of the adjustment is dependent upon the subjective 

 perception within much wider limits. Suppose two experienced 

 observers to make repeated measurements of one and the same 

 object with the same Microscope ; let the object be sharply 

 outlined and the measurements accurate to 1 mic. according to 

 the usual method of calculation : except by sheer chance, the 

 comparison of the mean values found will yield differences which 

 amount, with objects of 10 mic. diameter and upwards, to from 

 five to ten times the asserted error. This accuracy is therefore 

 not necessarily real. 



The mounting and arrangement of the eye-piece micrometer 

 admit of much variety. In small instruments it is generally 

 fixed immovably in the plane of the diaphragm ; hence it is 

 not equally clearly seen by eyes of different focal distance, 

 moreover, the adjustment of a division-line to a certain point 

 necessitates the displacement of the object. In this respect the 

 micrometers adapted to slide in the eye-piece mounting are some- 

 what more convenient; the same eye-piece is thus available for 

 ordinary observations by removing the micrometer. [The authors 

 here mention several micrometers constructed by Continental 

 opticians which possess more or less efficient means of adjust- 

 ment; but here again the matter is dealt with somewhat in- 

 completely from an English point of view, and has therefore been 

 omitted.] 



The determination of the size of microscopic objects which only 



