182 



PROCEEDINGS OF THE AMERICAN ACADEMY 



From a comparison of the separate values obtained by myself and 

 by Professor Morley, the following conclusions are drawn : — 



(a) By comparing the separate values of Bands I. and IV. of 

 Plate I., obtained with the eye-piece micrometer, with the corre- 

 sponding mean values, the average probable error of the measure of 

 a single space is found to be 19 ten-million ths of an inch, the great- 

 est deviation from the mean in 15G measures being 8 millionths of 

 an inch. 



(b) Comparing with the mean value, the separate results obtained 

 with the eye-piece micrometer, the Merz screw, and the filar microm- 

 eter, from the first and fourth bands of Plate I. we find the follow- 

 ing average deviations : — 



For the eye-piece micrometer, 17 ten-million ths of an inch. 

 For the Merz screw, 25 ten-millionths of an inch. 



For the filar micrometer, 18 ten-millionths of an inch. 



(c) Comparing the measures of the separate spaces made by Pro- 

 fessor Morley with my own, made in the first band of Plate I. with 



