70 THE MICROSCOPE. 



field were always verified. The following series is a fair average. 

 Rogers' plate, column one, gives five successive settings on the first 

 line; column two, settings on the two hundred and fifty-first line, 

 and three, settings on line one: 



Similar measures on Fasoldt's plate gave o.i inch = 2.550 

 turns, from which it would appear that there is a difference between 

 the two plates of one part in 850. Some days previously less care- 

 ful measures had given Rogers' 2.545 turns; Fasoldt, 2.550 turns. 

 The following day these were found: 



At apparent upper end of lines Fasoldt, 2.5467 



At apparent lower end of lines " 2.5456 



At transverse line " 2.547/ 



On the long finding lines - - - - - Rogers', 2.5457 

 It will be noticed that the measures on Fasoldt's plate on 

 dift'erent days are not so accordant as on Rogers', because the lines 

 are not quite as perfect. 



On the whole, therefore, no difference can be proven between 

 these two micrometers; this result is the more gratifying because of 

 the great errors that existed not long ago in micrometers from the 

 most noted opticians. Only a few years ago the only one to be 

 found at the University was two per cent, in error, and Mr. Rogers 

 has shown by numerous measures that such errors were frequent. 



Indeed every microscopist and physicist is indebted to Mr. 

 Rogers for the possibility of procuving micrometers whose errors 

 are less than can be detected by any instruments except the most 

 powerful; he has shown so freely his ruling machines and his stand- 

 ards of length, and in his writings has shown such knowledge of the 

 subject of metrology, at the same time avoiding any claim of 

 absolute accuracy in his rulings, which would inevitably be a false 

 claim, that we cannot help believing his work the most accurate 

 that is to be had. No higher praise can be given to rulings (as regards 

 accuracy) than to say they are as good as Rogers' ''standard." 

 Americans may well be proud of Rogers' micrometers and Fasoldt's 

 test plates. 



