Stoney — Appreciation of Ultra- Visible Quantities. 531 



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 ten kilometres distance (over six miles), the ordinate is 

 exactly one millimetre. 



4. And to reach an ordinate which is as long as an inch, we 

 should have to go to a distance of 254 kilometres from the apex — 

 about 158 miles, or across Ireland. 



However, in the study of molecular physics, we are dealing 

 with measures that are fractions of a micron ; so that the ten 

 metres of our gauge that are next its apex are enough for us to 

 retain. 



o 



III. — Relation of the Gtauge to Angstrom's Map. 

 The wave-lengths of light are the longest of the small quantities 

 with which we need concern ourselves ; and the gradient of the 

 gauge has been specially chosen to be convenient in measuring 



o 



them. For this purpose, lay the gauge on Angstrom's map of th^ 

 '• Spectre normal du Soleil, " making the points on the base line at 

 4, 5, and 6 metres from the apex of the gauge coincide with the 

 positions 4000, 5000, and 6000 on his map. This can be done,, 



o 



since Angstrom's scale is a scale of millimetres. 



Then the actual length of A (the wave-length-in-air) for each 

 ray represented in his map is the ordinate of the gauge {i. e. the 

 vertical distance from the horizontal base of the gauge up to its 

 sloping top) immediately over the line of the map representing the 

 ray.^ 



lY. — The "Minimum visibile." 



The minimum visibile {i. e. the smallest separation at which two 

 points must stand to admit of their being seen as two, by the help 



1 Eowland's great photograph of the solar spectrum is on a scale which is about 

 three times larger than that of Angstrom's map ; and, from the exigencies of the case, 

 the lengths of the degrees upon it differ slightly from strip to strip. To adapt a gauge 

 to it, begin by extending both ways the scale of the strip under examination till it 

 reaches zero in one direction and 10,000 in the other. Over the 10,000 mark erect a- 

 micron, and from the top of it draw the inclined plane to the zero mark. This is the 

 gauge whose ordinates will be the wave-lengths of the rays represented in that strip of 

 the map. Each strip will require its own gauge ; but none of them will be far from 

 30 metres long, so that they are about three times more acute than the proposed 

 standard gauge. 



SCIBN. PBGC. R.D.S., VOL. VII., PART V. 2 T 



