230 THE AMERICAN MONTHLY [October, 



Seeds differ so much that one would need to make many variations 

 in method to suit different cases ; but as a general method I have found 

 this to be a success, and I believe the histology of any see^ mav be 

 demonstrated by applying it. 



New Stage Micrometers. 



By E. M. nelson. 



[Read at the Royal Microscopical Society, May, 1890.] 



Messrs. Powell and Lealand have a new micrometer so excellently 

 ruled as to be worthy of remark. It comprises looths and i ,oooths of 

 an inch, and loths and looths of a mm., there being lo divisions of 

 each set ; the finer divisions of .001 in. and .01 mm. being placed in the 

 centre, the .01 in. being on the one side and the .1 mm. on the other, 

 respectively, a guiding line being ruled at right angles to them. The 

 lines are fine, 30^-770 ^^"> ^^^ ^^^ blackened in and mounted in balsam. 

 The lines are straight and evenly ruled. With regard to the spacing, 

 I have made exhaustive comparisons with fine micrometers by Rogers 

 and Zeiss, and some others not quite so perfect. Upwards of 240 screw 

 micrometer measurements were made, and the work carried on under 

 hypercritical conditions. An account of these may be of interest. 

 First, a magnification of 1,200 diameters by means of a suitable immer- 

 sion lens was employed for the finer ruling, and for the coarse a dry 

 ^ X 600 diameters ; the screw micrometer was on an independent mount- 

 ing. Care being taken with regard to the illumination, etc., a critical 

 image of the lines was obtained. The order in which the lines were 

 taken was from left to right, as seen in the instrument ; each interval 

 was then designated by consecutive letters of the alphabet. The inter- 

 vals were then most carefully wired, and each value set down under its 

 corresponding letter ; when the ten spaces were finished they were 

 meaned. 



It was then easy to see which interval differed from the mean, and to 

 calculate how much. In the same way comparison can be made with 

 any other scale, it matters not whether it is ruled in inches or mm. It 

 is most important that both the instrument and the observer be tested. 

 To this end I proceed as follows : The screw value of 20 intervals on a 

 badly ruled scale was written down as above, the paper was then put 

 away, and the operation performed again. 



On comparing the two papers, the screw values of seven intervals 

 were identical, 12 different by one division, and i by two divisions. 

 This error of two divisions occurred in the interval H, the first read- 

 ing being 1033, and the second 1031. On careful re-examination of 

 this interval, I came to the conclusion that the first reading was the bad 

 one, and that the true value was 103 1 or 1032. On substituting this 

 last value in both sets of readings, the 20 intervals meaned precisely 

 alike, viz., 1038. This forms a suitable illustration of the work. With 

 the exception, therefore, of the interval H, the screw readings may be 

 taken as true to zh I • The mean 1038 being the value in divisions of 

 the screw-head, for 50//, the value of one division consequently = 

 .000001897 in., or less than 1-500,000 in. 



This might be called " the constant of the instrument and observer." 



