36 



SiX LECTURES ON LIGHT. 



brought to a focus ; and paper placed at the 

 focus is caused to smoke and burn. This 

 may be done by our common camera with its 

 lens, an.d by a concave mirror of very mod- 

 erate power. 



We \vi!l now adopt stronger measures with 

 the radiation from the electric lamp. In this 

 camera of blackened tin is placed a lamp, in 

 all particulars similar to those already em- 

 ployed. But, instead of gathering up the 

 rays from a carbon-point by a condensing 

 tens placed in front of them, we gather them 

 up by a concave mirror, silvered in front, 

 and placed behind the carbons. By this 

 mirror we can cause the rays to issue through 

 the orifice in front, either parallel or conver- 

 gent. They are now parallel, and therefore 

 to a certain extent diffused. We place a 

 convex lens in the path of the beam ; the 

 light is converged to a focus, and at that 

 focus you s;e that paper is not only pierced 

 and a burning ring formed, but that it is in- 

 stantly set ablaze. Many metals may be 

 burned up in the same way. In our first lec- 

 ture the combustibility of zinc was mentioned. 

 Placing a strip of sheet-zinc at this focus, it 

 is instantly ignited and burns with its charac- 

 teristic purple flame. (In the annexed figure 

 m m f represents the concave mirror, L the 



4 " 



lens, at the focus C of which combustion is 

 effected). Dr. Scoresby succeeded in ex- 

 ploding gunpowder by the sun's rays con- 

 verged by large lenses of ice ; the same effect 

 may be produced with a small lens, and with 

 a terrestrial source of heat. In an iron mould 

 we have fashioned this beautiful lens of 

 transparent ice. At the focus of the lens I 

 place a bit of black paper, with a little gun- 

 cotton folded up within it. The paper ignites 

 and the cotton explodes. Strange, is it not, 

 that the beam should possess such heating 

 power after having passed through so cold a 

 substance ? 



In this experiment, you observe that, before 

 the beam reaches the ice-lens, it has passed 



' through a glass cell containing water. The 

 beam is thus sifted of constituents, which, it 

 permitted to fall upon the lens, would injure 

 its surface, and blur the focus. And this 

 leads me to say an anticipatory word regard- 

 ing transparency. In our first lecture we 

 entered fully into the production of colors 

 by absorption, and we spoke repeatedly 

 of the quenching of the rays of light. Did 

 this mean that the light was altogether 

 annihilated? By no means. It was sim- 

 ply so lowered in refrangibility as to 

 escape the visual range. // was converted 

 into heat. Our red ribbon in the green of the 

 spectrum quenched the green, but if suitably 

 examined its temperature would have been 

 found raised. Our green ribbon in the red 

 of the spectrum quenched the red, but its 

 temperature at the same time was augmented 

 to a degree exactly equivalent to the light ex- 

 tinguished. Our black ribbon, when passed 

 through the spectrum, was found competent 

 to quench all its colors ; but at every stage of 

 its progress an amount of heat was generated 

 in the ribbon exactly equivalent to the light 

 lost. It is only when absorption takes place 

 that Jieat is thus pi'oduced ; and heat is always 

 a result of absorption. 



Examine this water, then, in front of the 

 lamp, after the beam has passed a little time 

 through it : it is sensibly warm, and, if per- 

 mitted to remain there long enough, it may 

 be made to boil. This is due to the absorp- 

 tion by the water of a portion of the electric 

 beam. But a certain portion passes through 

 unabsorbed, and does not at all contribute to 

 the heating of the water. Now, ice is also 

 transparent to the latter portion, and there- 

 fore is not melted by it ; hence, by employ- 

 ing this particular portion of the beam, we 

 are able to keep our lens intact, and to pro- 

 duce by means of it a sharply-defined focus. 

 Placed at that focus, black paper instantly 

 burns, because the black paper absorbs the 

 light which had passed through the ice-lens 

 without absorption. In a subsequent lecture, 

 we shall endeavor to penetrate further into 

 the physical meaning of these and other simi- 

 lar actions. I may add to these illustrations 

 of heating power, the ignition cf a diamond 

 in oxygen, by the concentrated beam of the 

 electric lamp. The diamond, surrounded by 

 a hood of platinum to lessen the chilling due 

 to convection, is exposed at the focus. It is 

 rapidly raised to a white heat, and when re- 

 moved from the focus continues to glow like 

 a star. 



Placed in the path of the beam issuing from 

 our lamp is a cell with glass sides containing 

 a solution of alum. All the light of the beam 

 passes through this solution. The beam is 

 received on a powerfully converging mirror 

 silvered in front, and is brought to a focus by 

 the mirror. You can see the conical beam 

 of reflected light tracking itself through the 

 dust of the room. I place at the focus a 

 scrap of white paper : it glows there with 



