SIX LECTURES ON LIGHT. 



35 



moment that notion of gradual growth, 

 amelioration, and ascension, implied by the 

 term evolution, we might fairly conclude that 

 there a*-e stores of visual impressions awaiting 

 man far greater than those of which he is 

 now in possession. For example, here be- 

 yond the extreme violet of the spectrum there 

 is a vast efflux of rays which are totally use- 

 less as regards our present powers of vision. 

 But these ultra-violet waves, though incom- 

 petent to awaken the optic nerve, can so 

 shake the molecules of certain compound sub- 

 stances as to effect their decomposition. The 

 grandest example of the chemical action of 

 light, with which my friend Dr. Draper has 

 so indissolubly associated his name, is that of 

 the decomposition of carbonic acid in the 

 leaves of plants. All photography is founded 

 on such actions. There are substances on 

 which the ultra-violet waves exert a special 

 decomposing power ; and, by permitting the 

 invisible spectrum to fall upon surfaces pre- 

 pared with such substances, we leveal both 

 the existence and the extent of the ultra- 

 violet spectrum. 



This mode of exhibiting the action of the 

 ultra-violet rays has been long known ; in- 

 deed, Thomas Young photographed the ultra- 

 violet rings of Newton. We have now to 

 demonstrate their presence in another way. 

 As a general rule, bodies transmit light or 

 absorb it, but there is a third case in which the 

 light falling upon the body is neither trans- 

 mitted nor absorbed, but converted into light of 

 another kind. Professor Stokes, the occupant 

 of the Chair of Newton in the University of 

 Cambridge, one of those original workers 

 who, though not widely known beyond 

 scientific circles, realiy constitute the core of 

 science, has demonstrated this change of one 

 kind of light into another, and has pushed 

 his experiments so far as to render the invisi- 

 ble rays visible. 



A long list of substances examined by 

 Stokes when excited by the invisible ultra- 

 violet waves, have been proved to emit light. 

 You know the rate of vibration corresponding 

 to the extreme violet of the spectrum ; you 

 are aware that, to produce the impression of 

 this color, the retina is struck 789 millions of 

 millions of times in a second. At this point, 

 the retina ceases to be useful as an organ of 

 vision, for, though struck by waves of more 

 rapid recurrence, they are incompetent to 

 awaken the sensation of light. But, when 

 such non-visual waves are caused to impinge 

 upon the molecules of certain substances 

 on those of sulphate of quinine, for example 

 they compel those molecules, or their con- 

 stituent atoms, to vibrate ; and the peculiar- 

 ity is, that the vibrations thus set up are of 

 slower period than those of the exciting 

 waves. By this lowering of tr.e rate of vi- 

 bration through the intermediation of the sul- 

 phate of quinine, the invisible rays are ren- 

 dered visible. Here we have our spectrum, 

 and beyond the violet I place this prepared 



paper. The spectrum is immediately elonga- 

 ted by the generation of a new light beyond 

 the extreme violet. President Morton has 

 recently succeeded in discovering a substance 

 of great sensibility which he has named 

 Thalloie, and he has been good enough to 

 favor me with some paper saturated with a 

 solution of this substance. It causes a very 

 striking enlongation of the spectrum, the 

 new light generated being of peculiar bril- 

 liancy. To this change of the rays from a 

 higher to a lower refrangibility, Stokes has 

 given the name of Fluorescence. 



By means of a deeply-colored violet glass, 

 we cut off almost the whole of the light of 

 our electric beam ; but this glass is peculiarly 

 transparent to the violet and ultra-violet 

 rays. The violet beam now crosses a large 

 jar filled with water. Into it I pour a solu- 

 tion of sulphate of quinine : opaque clouds, 

 to all appearance, instantly tumble down- 

 wards. But these are not clouds : there is 

 nothing precipitated here : the observed ac- 

 tion is a > action of molecules, not of particles. 

 The medium before you is not a turbid me- 

 dium, for, when you look through it at a 

 luminious surface, it is perfectly clear. If we 

 paint upon a piece of paper a flower or a 

 bouquet with the sulphate of quinine, and ex- 

 pose it to the full beam, scarcely anything is 

 seen. But on interposing the violet glass, 

 the design instantly flashes forth in strong 

 contrast with the deep surrounding violet. 

 Here is such a design prepared for me by 

 President Morton with his thallene : placed 

 in the violet light it exhibits a peculiarly 

 vivid and beautiful fluorescence. From the 

 experiments of Dr. Bence Jones, it would 

 seem that there is some substance in the hu- 

 man body resembling the sulphate of quinine, 

 which causes ail the tissues of the body to be 

 more or less fluorescent. The crystalline 

 lens of the eye exhibits the effect in a very 

 striking manner. When I plunge my eye 

 into this violet beam, I am conscious of a 

 whitish-blue shimmer filling the space before 

 me. This is caused by fluorescent light gen- 

 erated in the eye itself ; looked at from with- 

 out, the crystalline lens at the same time 

 gleams vividly. 



But the waves from our incandescent car- 

 bon-points appeal to another s^nse than that 

 of vision. They not only produce light as a 

 sensation; they also produce heat. The mag- 

 nified image of the carbon-points is now upon* 

 the screen, and with a suitable instrument the 

 heating power of that instrument might be 

 demonstrated. Here, however, the heat is 



pread over too large an area to be intense. 

 By pushing out the lens and causing a mova- 

 ble screen to approach our lamp, the imige 

 becomes smaller and smaller : the rays be- 

 come more concentrated, until finally they 

 are able to pierce black paper with a burning 



ing. Rendering the beam parallel, and re- 

 ceiving it upon a concave mirror, the rays are 



