468 Analyses of Books. [Dec. 



sharp edge is the summit of the crystal. The broad faces are inclined 

 upon the adjacent faces at an angle of 64". The crystal is terminated 

 by four minute planes, equally inclined to the broad face and the axis 

 of the prism, but two of the faces often disappear. The crystals are 

 generally opaque, and at thicknesses not much above the 2V th of an 

 inch ; they are quite impervious to the sun's rays. Their colour by 

 reflected light is then nearly black ; but their powder, in daylight, 

 is green, and French ^ray by candle light. In the smaller crystals 

 te colour is blue, both by reflected and transmitted light. The 

 refractive index is about 1*605 and 1*506. At a certain small thick- 

 ness the least refracted image is bright blue, and the most refracted 

 image bright green in daylight, or bright pink in candle light. 

 The blue, when analyzed, consists of a mixture of green, and the 

 green an admixture of red. 



At greater thicknesses the blue becomes purer and fainter, and the 

 green passes into red ; and at a certain thickness the least refracted 

 blue image disappears altogether, and the most refracted image is_ 

 olive-green. At still greater thicknesses this image disappears also, 

 and absolute opacity ensues. With polarized light, when the axis 

 of the crystal is in the plane of polarization, the transmitted light is 

 green, but when perpendicular it is blue. In solution the double 

 refraction disappears, but the other appearances are observed as in 

 the solid state. The crystal excites a specific action on the red ray 

 between A & B of Fraunhofer ; a sharp and narrow black band being 

 formed, which constitutes a fixed line in all artificial lights. The 

 relations of this salt to common and polarized light, may be examined 

 by placing upon a plate of glass a few drops of a saturated solution 

 of it in water. If the crystals are slowly formed they will be found 

 of various thicknesses, each thickness exhibiting a different colour, 

 varying from perfect transparency through all shades oi pale -yellow, 

 green, and blue^ in daylight, and through all shades of pale-yellow, 

 pale-orange, red, and blue in candle-light. 



Second Essay on a General Method in Dynamics. By W. R. 

 Hamilton. 



In his First Essay, the author observed, that many eliminations 

 required by this method, in its first conception, might be avoided by 

 a general transformation, introducing the time explicitly into a part 

 S of the whole characteristic function V. In the present Essay he 

 fixes his attention chiefly on this part S, and to call it the Principal 

 function. Its properties are more fully developed, especially in 

 application to questions of perturbation, in which it enables us to 

 express accurately the disturbed configuration of a system by the 

 rules of undisturbed motion, if only the mutual components of velo- 

 cities be changed in a suitable manner. 



Continuation of a paper on the twenty-five feet Zenith Telescope, 

 lately erected at the Royal Observatory. By John Pond. 



