Appreciation of Ultra-visible Quantities. 421 



rows. It is the ordinate of our gauge at about six and a half 

 metres from its end, and is the wave-length of a ray of red 

 light not far from the red hydrogen-line, the line C of the 

 solar spectrum ; so that the brown dots succeeding one 

 another in a row mark off in the field of the microscope the 

 successive waves of this particular ray of light. 



The dots are arranged in rows parallel to the sides of an 

 equilateral triangle, and with oblique light conrng at right 

 angles to any one of these sets of rows, the dots will elongate 

 and almost run into one another in a way that makes the 

 rows look like a ruling of parallel lines. These parallel lines 

 are at shorter distances asunder than the dots in the ratio of 

 \/3 to 2, and accordingly present to the eye intervals equal 

 to the wave-length of a green ray less refrangible than the 

 line E of the solar spectrum. The interval in this case is the 

 ordinate of our gauge at a distance of about five metres and a 

 third from its apex. 



Furthermore, what we have found above to be the minimum 

 visibile is a little more than one third of the interval from 

 centre to centre of the dots, or a little less than half the 

 interval of the rows. It is well illustrated by the Pleuro- 

 sigma markings. In fact, judging from similar markings on 

 other scales, the round dots would be seen as rings were it 

 not for their small size, which prevents the opposite sides of 

 the ring from being seen as two objects. They accordingly 

 look like disks *. 



VIII. Of the Nomenclature of Small Measures* 

 It will often be found convenient to connect the proposed 

 standard gauge with another useful way of describing small 

 magnitudes. Let us understand by a sixthet a unit in the 

 sixth place of decimals, i. e. the fraction of 1/10 6 , and let us 

 use the phrase sixthet-metre, or metre-sixthet, to mean the 

 sixthet of a metre, in the same sense in which we say half- 

 inch or quarter-inch to mean the half or quarter of an inch. 



We can then conveniently express the following table of 

 equivalents. 



The ordinate of the standard gauge, at a distance : — 

 Of ten metres from the apex = a sixthet-metre. 

 Of one metre „ ,, = a seventhet-metre. 

 Of one decimetre „ „ = an eighthet-metre. 

 Of one centimetre „ = a nintheo-metre. 



Of one millimetre „ = a tenthet-metre. 



* The white pearl-like specks which take the place of the dots when 

 they are a little out of focus, must not be mistaken for their being seen 

 as rings. They are an optical effect, and of larger size 



Phil. Mag. S. 5. Vol. 34. No. 210. Nov. 1892. 2 G 



