CHAMBERS'S INFORMATION FOR THE PEOPLE. 



microscope magnifies 720 diameters. Good in- 

 struments are usually furnished with three eye- 

 pieces of different powers, and five or six object 

 combinations, of focal lengths varying from one 

 inch to the twelfth of an inch (or less), so that 

 magnifying power may range from 20 to 2000 

 diameters. 



INTERFERENCE, DOUBLE REFRACTION, AND 

 POLARISATION OF LIGHT. 



Interference. Suppose that in consequence of 

 a disturbance, a series of up-and-down waves is 

 propagated through an elastic medium, and that, 

 from a second disturbance, another set of the same 

 wave-length is also propagated through it ; it is 

 easy to see that the total disturbance at any point 

 will depend on the state or phase in which each 

 wave reaches it. If the crests of both waves reach 

 the point at the same time, it will be displaced 

 through a distance equal to the sum of the separate 

 displacements ; and as both waves travel at the 

 same rate, the effect is the same as if one set of 

 larger waves were passing. But if the crest of the 

 first wave should reach the point along with the 

 hollow of the second, the total displacement is 

 the difference of the two ; and if these are equal, 

 the point remains at rest. In these cases, the 

 waves are said to interfere with each other ; and 

 we see that one wave will absolutely destroy an 

 equal one, if their phases differ by half a wave- 

 length, or a length and a half, two and a half, &c. 

 As light consists of waves, it should be possible 

 for two lights to produce darkness, and this 

 strange conclusion has been established by direct 

 experiment. Let A and B be two small holes, 

 very close together, in the shutter of a darkened 

 room, through which a homogeneous light is 



Fig- So- 

 admitted. If a screen, of which EC is the section, 

 be held at a suitable distance to receive the 

 diverging pencils, it will be found that the intensity 

 of the light in EC is not uniform, but that there 

 will be a bright spot at C, which is equally distant 

 from the two holes, a dark spot at D, a white 

 spot at E, and so on. These spots on the sec- 

 tion EC are of course bands on the screen; and 

 if one of the holes, as B, be stopped, the bands 

 suddenly disappear; if it be opened again, they 

 reappear. Thus, the addition of more light to D 

 has caused darkness. By careful measurement, 

 it has been found that the difference between the 

 paths AD and B D is half a wave-length ; between 

 AE and BE, two halves ; and so on. If the com- 

 pound light of the sun be used, the blue, which 

 has the smallest wave-length, occupies a dif- 

 ferent position at E from the red, whose wave- 

 length is greatest.; and the other colours lie be- 

 tween. There is thus a coloured band at E, with 

 the blue lowest ; and these bands are repeated 



253 



until the colours of one band are mingled with 

 those of the next, and the screen is white. 



Double Refraction. If a ray of light be allowed 

 to fall on a rhomb of Iceland spar, instead of being 

 refracted as in glass, it is split up into two rays of 

 equal intensity, one of which obeys the ordinary 

 law of refraction, and the other does not. This 

 property of double refraction belongs to all crystal- 

 lised minerals, except those of which the cube is 

 the fundamental form, or, more generally, to all 

 bodies whose density differs in different directions. 

 Thus, a cube of glass can be made to refract 

 doubly, by compressing it strongly in one direc- 

 tion. The two rays into which the single one has 

 been split up do not at first seem to differ from 

 each other, or from common light ; but if they be 

 allowed to fall on a second rhomb, a remarkable 

 difference is observed. If, for shortness' sake, we 

 designate the ordinary and extraordinary rays by 

 the letters O and E, we find that though in 

 general both O and E are split up into two 

 others, OO, OE, EO, EE, the two components 

 are no longer of the same intensity, and that in 

 certain relative positions of the crystals, one dis- 

 appears altogether. If the two crystals are in 

 similar positions, O produces only OO, OE having 

 quite disappeared, and E produces only EE, EO 

 having disappeared. If the second crystal be 

 turned through 90, OE and EO alone remain. 

 Thus, not only have the two rays, O and E, 

 acquired properties with respect to space (that 

 is, got sides or poles), but these properties are the 

 same for the two rays with respect to two planes 

 at right angles to each other. The two rays are 

 said to be polarised, and their planes of polarisa- 

 tion are perpendicular to each other. 



Polarised Ligkt. Some crystals, after separat- 

 ing the two rays, absorb one of them, as tourmaline, 

 which suppresses the ordinary ray. There are 

 other ways of obtaining one ray without the pres- 

 ence of the other, but this method will be assumed 

 for simplicity. If we wish to examine any pencil 

 of light to ascertain whether or not it is polarised, 

 we must use a double refracting rhomb, or such an 

 analysing plate. If we find that in a certain 

 position of the analyser, one ray is extinguished, 

 the beam was wholly polarised; but if the intensity 

 was only weakened, then the polarisation was 

 partial. 



Some of the most splendid phenomena in optics 

 are produced when bodies are examined by polar- 

 ised light. Let T be the polariser, a plate of 



Fig. 31- 



tourmaline cut parallel to its axis ; R, the analysing 

 rhomb ; and C, a doubly refracting crystal, which 

 we are examining. A ray of common light, A, 

 after passing through T, is polarised, its companion 

 ray having been quenched. B is split up by C 

 into E and O, polarised at right angles to each 

 other, and differing in phase, E being retarded 

 more than O. If E and O were common light, they 



