SUNLIGHT, SEA, AND SKY. S i 



To which of these two ideals I myself lean has perhaps already be- 

 trayed itself; and that being so, I shall venture to consider your pres- 

 ence here a pi-oof that, for this evening at least, you side with me, 

 and that you are willing to spend an hour of your leisure in an intel- 

 lectual effort to see a little deeper into those phenomena which Nature 

 in this place and at this season displays with such profusion and 

 splendor.' 



But at the outset I must warn you that we are met by a difficulty, 

 for the surmounting of which you must rely upon yourselves rather 

 than upon me. It is this : the phenomena to which I propose to draw 

 your attention, although taking place nearly every day, and all day 

 long, and in almost every direction, are veiled from our eyes ; and it is 

 only by the use of special appliances to aid our eyes that they can be 

 made visible. It will be my business to supply these appliances, and, 

 reproducing on such scale as may be possible within these four walls 

 the optical processes which are going on in the sea and sky outside, to 

 exhibit the hidden phenomena of which I am speaking. But it must 

 be your part to transport yourselves mentally from the mechanism of 

 the lecture-room to the operations of Nature, and by a " scientific use 

 of the imagination " (to adopt what has now become a household word 

 at these meetings) to connect the one with the other. 



Now the main point in question is this : that light, when subjected 

 to the very ordinary processes of reflection from smooth surfaces, such 

 as a window, a mahogany table, or the sea itself, or when scattered to 

 us from the deep clear sky, undergoes in many cases some very pe- 

 culiar changes, the character and causes of which we have come here 

 to investigate. The principal appliance which will be used to detect 

 the existence of such changes, as well as to examine their nature, con- 

 sists of this piece of Iceland sj>ar, called from the man who first con- 

 structed a compound block of the kind a Nicol's prism, and this plate 

 of quartz or rock crystal ; both of which, as you will observe when 

 the light passes through them, are clear, transparent, and colorless, 

 and both of which transmit the direct light from the electric lamp with 

 equal facility, however they may be turned round about the beam of 

 light as an axis. 



If, however, instead of allowing the beam to fall directly upon the 

 Nicol, we first cause it to be reflected from this plate of glass, we shall 

 find that the process of reflection has put the light into a new condi- 

 tion. The light is no longer indifferent to the rotation of the Nicol ; 

 in one position of the Nicol the light passes as before, but as the in- 

 strument is turned round the light gradually fades, and when it is 

 turned through a right angle the light is extinguished. Beyond this 

 position the light reappears, and the same changes of fading and re- 

 vival are observed in the light for every right angle through which 

 the instrument is turned. 



But these phenomena are susceptible of a very beautiful modifica- 



