Presidential Address. 9 



the tube. You will notice that the tube appears empty for a 

 moment after the turning on of the beam ; but the chemical action 

 will be so rapid that attention is requisite to mark this interval of 

 darkness. I ignite the lamp, and a luminous white cloud immedi- 

 ately falls upon the beam. The beam has, in fact, shaken asunder 

 the molecules of the nitrite of amyl, and brought down upon itself 

 a shower of particles which flash forth like a solid luminous spear. 

 This experiment, moreover, illustrates the fact that however intense- 

 a beam of light may be it remains invisible unless it has something 

 to shine upon. Space, though traversed by the rays from all sun& 

 and all stars, is itself unseen. Not even the ether which fills space,, 

 and whose motions are the light of the universe, is itself visible. 



It is possible to impart to these clouds any required degree of 

 tenuity, for it is in our power to limit at pleasure the amount of 

 vapour in our *experimental tube. When the quantity is duly 

 limited, the precipitated particles are at first inconceivably smalL 

 defying the highest microscopic power to bring them within range 

 of vision. Probably their diameters are then not greater than the- 

 millionth of an inch. They grow gradually, and as they augment 

 in size they throw from them a continually increasing quantity of 

 wave-motion, until finally the cloud which they form becomes so 

 luminous as to fill a room with light. During the growth of the 

 particles the most splendid iridescences are often exhibited. It is- 

 not however with the iridescences, however beautiful they may be,, 

 that we have now to occupy our thoughts, but with other effects- 

 which bear upon the great standing enigmas of meteorology — the 

 colour of the sky and the polarization of its light. 



The blue light of the sky is scattered light ; and were there 

 nothing in our atmosphere competent to scatter the solar rays, we 

 should see no blue firmament, but the mere darkness of infinite 

 space. The blue of the sky is produced by perfectly colourless- 

 particles. Smallness of size alone is requisite to ensure the selection 

 and reflexion of this colour. 



It is possible, as above stated, by duly regulating the quantity 

 of vapour, to make our precipitated particles grow from an infini- 

 tesimal and altogether ultra-microscopic size to specks of sensible 

 magnitude ; and by means of these particles, in a certain stage of 

 their growth, we can produce a blue which shall rival, if it does not 

 transcend, that of the deepest and purest Italian sky. 



Here, then, we have the fundamental principles governing ultra- 

 microscopic methods and processes, a truly remarkable example of 

 scientific originality and foresight. 



We know in practice that under conditions which secure the same 

 magnification, the same methods of illumination, and similar cor- 

 rections of aberrations in the objective, the best definition and highest 

 resolution is always secured by the lens with the highest numerical! 

 aperture. Further, we realize that a reduction in the mean wave- 



