94 American Quarterly Microscopical Jourtial. 



posed for this paper by comparing the results of two inde- 

 pendent observers. The writer made many measurements in 

 duplicate, in order to examine the same question by compar- 

 ing the independent results of the same observer. 



The measurements made for this purpose are one hundred 

 and twenty-three in number. Twenty-three were made with 

 a glass of one inch nominal focal length, whose real focal length 

 is eighty hundredths of an inch. The other measurements 

 were made with a glass called a quarter-inch, whose real 

 focal length is twenty hundredths of an inch. The microm- 

 eter used was a cobweb micrometer, by Troughton & Simms. 

 It has two movable wires; the heads of the screws are 

 divided into one hundred parts, and were commonly read to 

 the fourth part of a division. The screws have threads of 

 about one hundred and three to the inch. 



Care was taken to keep the distance between the objective 

 and micrometer constant, by not moving the fine adjustment 

 of the microscope. Before noting measurements, preliminary 

 measurements were made until it was thought that parts of 

 the instrument, warmed by the approach of the observer's 

 person, had assumed a new equilibrium. Great care was taken 

 that the second of two measurements of the same space 

 should be free from any influence of bias in favor of the 

 value found by the first. When the two measurements were 

 made with no other measurements intervening, the heads of 

 the micrometer screws were disturbed at random, and were 

 not seen again until the contact between the wires and the 

 images of the lines had been finally established; and the re- 

 sult was, in all cases but one, set down with no correction or 

 modification whatever, either of the second or of the first re- 

 sult. Since there is scarcely any room for supposing that any 

 bias would affect the reading of the screw heads to the fourth 

 part of a division, it is difficult to see why two results thus 

 obtained are not, substantially, as independent as would be 

 the results of two observers, sitting in succession at the same 

 instrument, and possessed of the same habits of obsei"vation, 

 but ignorant of each other's results. 



When the lines measured were such as to give a sharply 

 defined margin with the power used, the wires of the microm- 

 eter were brought gradually nearer the right-hand margin of 

 the two lines, until the bands of light, left intervening be- 



