[ 441 J 



XXX. 



A METHOD OF MICEOSCOPIC MEASTJEEMENT. 



By J. JOLT, ScD., F.E.S., 



Professor of Geology and Mineraology in the University of Dublin. 



[Head December 17, 1912. Published February 7, 1913.] 



Eecently having to measure the diameter of some small objects in a rock- 

 slice, I was led to use the following method : — 



Two fine lines are drawn with a drawing-pen in indian 

 ink on a piece of white paper. Tlie lines meet at a point, 

 and very slowly diverge. In this particular case the separa- 

 tion of the lines was about 5 millimetres at a distance of 

 10 centimetres from the point of intersection. 



The camera-lucida is placed in position, and the image 

 of the object under the microscope referred in the usual 

 manner to the slieet of white paper. 



The paper is then shifted till the object appears to fit 

 exactly between the lines. While still looking at the object 

 a mark is made with a pencil across the lines just where the 

 object is referred. 



An engraved scale, divided to O'Ol of a millimetre, is now 

 substituted for the object, and a few — say, n — of the sub- 

 divisions brouglit by the camera-lucida to fit — as before — 

 between the lines. This point is marked on the paper as 

 before. 



The diameter of the object is now found by measuring 

 the distance di from the intersection of the lines at which the 

 first pencil-mark was made, and the distance d^ at which the 

 second pencil- mark was made. This is done simply by measure- 

 ment with a millimetre scale. Then the diameter, *, of the 

 object is evidently found from the proportion 



X : 11 : : di : di. 



This method is quicker and more accurate than endeavour- 

 ing to draw the -object by the use of the camera-lucida, and 

 then measuring the drawing. It is,, I found in my own case, 

 more consistent in successive measurements tliau the micrometer eye-piece, 

 and of course, the only apparatus required is the camera-lucida. 



SCIEKT. PKOO. E.D.S., VOL. XIII., NO. XXX. 3 U 



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