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XXXVII. 



ON THE APPEECIATION OF ULTEA- VISIBLE QUANTITIES, 

 AND ON A GAUGE TO HELP US TO APPEECIATE 

 THEM. By G. JOHNSTONE STONEY, M.A., D.Sc, 

 F.E.S., Vice-President, Eoyal Dublin Society. 



[Read March 16, 1892.] 



I. — Description of the Gtauge. 



Imagine a quadrant of the earth's meridian to be straightened out, 

 and used as the base line of a wedge-shaped gauge. Set a metre 

 upright at one end of this base, and from the top of it draw the 

 inclined plane to the other end. This completes the gauge. It is, 

 in fact, a wedge with a slope of one in 10,000,000. We shall 

 only require the last ten metres of this gauge, next its apex ; and 

 it is tliis portion which I propose as a standard for the measurement 

 of small quantities. Small quantities are to be measured by the 

 ordinates of the gauge, that is by the little perpendicular distances 

 from its base line up to its sloping top. 



Another and perhaps a better way of conceiving the gauge is 

 to take a base line that is only ten metres long, to erect a micron^ 

 at one end, and from the top of this to draw the incline to the 

 other end. This will give the same slope as before — a gradient of 

 one in 10,000,000. 



II. — Illustrations or the very Acute Angle of this 



Gtauge. 



1. A wedge with an angle of 1" would furnish a slope of one 

 in 206,265. Ours has a slope of only one in 10,000,000. It is, 

 accordingly, between 48 and 49 times more acute : in other words, 

 its angle is less than the forty-eighth of I", which is a much smaller 

 angle than can be measured by any astronomical instrument. 



1 The micron is a measure that has come of late years into general use among 

 microscopists. It is the thousandth of a millimetre, which is the same as l/25400th 

 of an inch. The micron is between the seventh and the eighth part of the diameters 

 of the little red corpuscles in human blood, which are tolerably uniform in size and are 

 familiar objects to all workers with the microscope. 



