164 PROCEEDINGS OP THE AMERICAN ACADEMY 



XII. 



ON THE LIMITS OF ACCURACY IN MEASUREMENTS 

 WITH THE MICROSCOPE. 



By Professok Edward W. Morley, of Western Reserve College. 



Presented Oct. 9, 1878. 



The following measurements of rulings on glass, by Mr. Rogers, were 

 made with an objective of two tenths of an inch focus, and a cobweb 

 micrometer. For a description of the ruled plates the reader is re- 

 ferred to page 178 of the present volume of the Proceedings. Light 

 was thrown on the rulings by reflection from clouds : care was taken to 

 have the light as uniform as possible. Such care is necessary in mak- 

 ing accurate measurements with a lens of short focus. The screw for 

 fine adjustment was permitted to be moved only through half a revo- 

 lution during the measurements. The same parts of the micrometer 

 screw were used throughout the measurements of a band. The image 

 of the line ruled on the plate consists of a bright central line, with a 

 darker line on each side ; the wires of the micrometer were placed on 

 this central brighter line, and so near its apparent left-hand limit that 

 the bright line included between the dark wire and the dark border of 

 the image of the ruled line was the minimum visible quantity, and 

 was the same for both wires. Care was taken not to look at the index 

 of the micrometer until the coincidence of the wires was finally estab- 

 lished ; and also to move the wires a considerable quantity before 

 making a second measurement, except in perhaps five cases on the 

 third plate. In two cases the coincidence thus finally established was 

 re-examined after the reading had been taken, on account of divergence 

 from a previous result, and in one of these the coincidence was found 

 to be imperfect. With this exception, the figures given are absolutely 

 the whole of the measurements on the rulings. 



