SIX LECTURES ON LIGHT. 



the air, we may also have combustion it* a 

 liquid. Watr, for example, contains a store 

 of oxygen which may unite with and consume 

 a metal immersed in it. It is from this kind 

 of combustion that we are to derive the heat 

 and light employed in the present course. 



Their generation merits a moment's atten- 

 tion. Before you is an instrument a small 

 voltaic battery in which zinc is immersed in 

 ! a suitable liquid. Matters are so arranged 

 that an attraction is set up between the metal 

 and the oxygen, actual union, however, being 

 in- the first instance avoided. Uniting the 

 two ends of the battery by a thick wire, the 

 attraction is satisfied, the oxygen unites with 

 the metal, the zinc is consumed, and heat, as 

 usual, is the result of the combustion. A 

 power, which, for want of a better name, we 

 call an electric current, passes at the same 

 time through the wire. 



Cutting the thick wire in two, I unite the 

 severed ends by a thin one. It glows with a 

 white heat. Whence comes that heat ? The 

 question is well worthy of an answer. Sup- 

 pose in the first instance, when the thick wire 



to form the image. It H not sharp, but sur, 

 rounded by a halo whic'i nearly obliterates it. 

 This arises from an imperfection of the lens, 

 called its spJierical aberration, due to the fact 

 that the circumferential and central rays have 

 not the same focus. The human eye labors 

 under a similar defect, and, when you looked 

 at the naked light from fifty cells, the blur of 

 light upon the retfna was sufficient to destroy 

 the definition of th<; retinal image of the car- 

 bons. A long list of indictments might in- 

 deed be brought against the eye its opacity, 

 its want of symmetry, frs lack of achroma- 

 tism, its absolute blindness, in pa;t. All 

 these taken together caused an eminent Ger- 

 man philosopher to say that, if any optician 

 sent him an instrument so full of defects, he 

 would send it back to him with the severesf 

 censure. But the eye is not to be judged 

 from the standpoint of theory. As a practi' 

 cal instrument, and taking the adjustments by 

 which its defects are neutralized into account, 

 it must ever remain a marvel to the reflecting 

 mind 



Tae ancients, as I have said, were aware o/ 



was employed, that we had permitted the ac- | the rectilineal propagation of light. They 



lion to continue until 100 grains of zinc were 

 consumed, the amount of heat generated in 

 the battery would be capable of accurrte nu- 

 merical expression. Let the action now con- 

 tinue, with this thin wire glowing, until 100 



knew that an opaque body, placed between 

 the eye and a point of light, intecepted the 

 light of the point. Possibly the terms " ray " 

 and "beam" may have been suggested by 

 those straight spokes of light which, in c^r 



grains of zinc are consumed. Will the amount j tain states of the atmosphere, dart from thi 



t-f heat generated in the battery be the same 

 as before ? No, it will b>i less by the precise 

 amount generated in the thin wire outside the 

 battery. In fact, by adding the internal heat 

 to the external, we obtain for the combustion 

 of 100 grains of zinc a total which never va- 

 ries. By this arrangement, then, we are able 

 to burn our zinc at one place, and to exhibit 

 the heat and light of its combustion at a dis- 

 tant place. In New York, for example, we 

 have our grate and fuel ; but the heat and 

 light of our fire may be made to appear at 

 San Francisco. 



I now remove the thin wire and attach to 

 the severed ends of the thick one two thin 

 rods of coke. On bringing the rods together 

 we obtain a small star of light. Now, the 

 light to be employed in our lectures is a sim- 

 ple exaggeration of this star. Instead of 

 being produced by ten cells, it is produced by 

 fifty. Placed in a suitable camera, provided 

 with a suitable lens, this light will give us all 

 the beams necessary for our experiments. 



And here, in passing, let me refer to the 



sun at his rising and his setting. The recti-V 

 lineal propagation of light may be illustrated 

 at home in th : s way: Make a small hole in a 

 closed window-shutter, before which stands a 

 house or a tree, and place within the dark- 

 ened room a white screen at some distance 

 from the orifice. Every straight ray proceed- 

 ing from the house or tree stamps its color 

 upon the screen, and the sum of all the rays 

 forms an image of the object. But, as the 

 rays cross each other at the orifice, the image 

 is inverted. Here we may illustrate the sub- 

 ject thus: In front of our camera is a large 

 opening, closed at present by a sheet of tin- 

 foil. Pricking by means of a common sew- 

 ing-needle a small aperture in the tin-foil, an 

 inverted image of the carbon-points starts 

 forth upon the screen. A dozen apertures 

 will give a dozen images, a hundred a hun- 

 dred, a thousand a thousand. But. as the 

 apertures come closer to each other, that is to 

 say, as the tin-foil between the apertures van- 

 ishes, the images overlap more and more. 

 Removing the tin-foil altogether, the screen 



common delusion that the works of Nature, I becomes uniformly illuminated Hence the 

 the human eye included, are theoretically per- | light upon the screen may be regarded as the 



overlapping of innumerable images of the 

 carbon-points. In like manner the light 

 upon every white wall on a cloudless day 

 may be regarded as produced by the super- 

 position of innumerable images of the sun. 



The law that the angle of incidence is 

 equal to the angle of reflection is illustrated 



The degree of perfection of any organ 

 is determined by what it has to do. Looking 

 at the dazzling light from our large battery, 

 you see a globe of light, but entirely fail to 

 see the shape of the coke-points whence the 

 light issues. The cause may be thus made 

 clear : On the screen before you is projected 



an. image of the carbon-points, the whole of j in this simple way: A straight lath is placed 

 the lens in front of the camera being employed I as an index perpendicular to a small looking- 



