416 Dr. G. J. Stoney on the 



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. 



III. Relation of the Gauge 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 con- 

 venient in measuring them. For this purpose, lay the gauge 

 on Angstrom's map of the " 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, since Ang- 

 strom's scale is a scale of millimetres. 



Then the actual length of \ (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*. 



IV. 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 of such coarse waves as the waves of light) is about 

 half the wave-length of the light admitted to our microscope; 

 that is, it is the ordinate of our standard gauge at some point 

 between two and three metres from its apex. All smaller 

 magnitudes are ultra-visible. 



V. The Larger Ultra-visible Magnitudes. 



1. Ponderable matter is in the gaseous state when its 

 molecules are so little crowded that they have room to dart 



* Rowland'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 ordinstes 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 timts more acute than the proposed standard gauge. 



