NOTE:?. 431 



changes successively to those given in figs. 68, 69, and 70. The colors 

 of the rings are the same with those of thin plates, but they vary with 

 the thickness of the nitre. Their breadth enlarges or diminishes also 

 with the color, when homogeneous light is used. 



NOTE 209, p. 183. Fig. 71 represents the ap- Fig. 71. 



pearance produced by placing a slice of rock 

 crystal in the polarized ray rs, fig. 64. The 

 uniform color in the interior of the image de- 

 pends upon the thickness of the slice ; but 

 whatever that color may be, it will alternately 

 attain a maximum brightness and vanish with 

 the revolution of the glass B. it may be ob- 

 served, that the two kinds of quartz, or rock 

 crystal, mentioned in the text, are combined in 

 the amethyst, which consists of alternate layers 

 of right-handed and left-handed quartz, whose 

 planes are parallel to the axis of the crystal. 



NOTE 210, p. 187. Suppose the major axis A P of an ellipse, fig. 18, to 

 be invariable, but the eccentricity C S continually to diminish, the' 

 ellipse would bulge more and more ; and when C S vanished, it would 

 become a circle whose diameter is A P. Again, if the eccentricity were 

 continually to increase, the ellipse would be more and more flattened till 

 CS was equal to CP, when it would become a straight line A P. The 

 circle aud straight line are therefore the limits of the ellipse. 



NOTE 211, p. 187. The colored rings are produced by the interference 

 of two polarized rays : .n different states of undulation, on the principle 

 explained for common light. 



NOTE 212, p. 217. If heat from a non-luminous source be polarized fey 

 reflection or refraction at r, fig. 64, the polarized ray r s will be stopped 

 or transmitted by a plate of mica M I under the same circumstances that 

 it would stop or transmit the light ; and if heat were visible, images anal- 

 ogous to those of figs. 65, 67, &c.~would be seen at the point s. 



NOTE 213, p. 219. The Rev. John Buchanan, of Charleston, South 

 Carolina, has recently shown, by ingenious experiments, that the vulture 

 is directed to his prey by the sense of sight alone. 



NOTE 214, p. 267. The class Cryptogamia contains the ferns, mosses, 

 funguses, and sea-weeds : in all of which the parts of the flowers are 

 either little known or too minute to be evident. 



NOTE 215, p. 269. Zoophites are the animals which form madrepores, 

 corals, sponges, &c. 



NOTE 216, p. 269. The Saurian tribes are creatures of the lizard or 

 crocodile kind. Some of those found in a fossil state are of enormous size. 



P 



NOTE 217, p. 315. When a stream f|, Fig. 72. 



of positive electricity descends from P 

 to n, fig. 72, in a vertical wire at right 

 angles to the plane of the horizontal ; * 



circle A B, the negative electricity as- 

 cends from n to P, and the force ex- 

 erted by the current makes the north 

 pole of a magnet revolve about the * 

 wire in the direction of the arrow- 

 heads in the circumference, and it 

 makes the south pole revolve in the 

 opposite direction. When the current 

 of positive electricity flows upward 

 from n to P, these effects are reversed. 



