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XLIX. On the Appreciation of Ultra-visible Quantities, and 

 on a Gauge to kelp us to appreciate them. By GI-. John- 

 stone Stoney, M.A.j D.Sc, F.R.S., Vice-President, Royal 

 Dublin Society*. 



I. Description of the Gauge. 



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 ten millions. We shall only require the last 

 ten metres of this gauge, next its apex ; and it is this portion 

 which I propose as a standard for the measurement of small 

 quantities. Small quantities are to be measured by the ordi- 

 nates 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 f 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 of the very Acute Angle of this Gauge. 



1. A wedge with an angle of V 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 1 ;/ , 

 which is a much smaller angle than can be measured by any 

 astronomical instrument. 



2. Prolong the gauge beyond the ten metres. Then the 

 slightly differing diameters of the red corpuscles in human 

 blood are equal to the ordinates of the gauge at from 70 to 

 80 metres from its apex — about as far as street-lamps are 

 from one another. 



3. At 10 kilometres distance (over six miles) the ordinate 

 is exactly one millimetre. 



* From the l Scientific Proceeding's ' of the Royal Dublin Society, 

 vol. vii. p. 530. Communicated by the Author. 



•f 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. 



