556 FOURTH REPORT — 1834. 



be the hundred and ten milhonth of direct sunshine, which as- 

 suredly would illude all vision; and as the moon's latitude at 

 the middle of the eclipse was only four minutes, a considerable 

 part of its disc ought to have been dark. 



The only part of this reasoning which may be questioned is 

 the coefficient e. It was derived by Laplace from Bouguer's 

 experiments, and we cannot but feel that we have a right to 

 demand something better from the improved powers of mo- 

 dern reseai'ch. The actinometer, described at the last Meeting 

 of the Association, (see Reports of the Cambridge Meeting, 

 p. 379,) promises such a result, which, if possible, should be ob- 

 tained at different elevations, and accompanied with determina- 

 tions of the density and hygrometric state of the air. In the 

 ascent of Gay Lussac he did not pass the region of clouds ; but 

 even at heights much inferior, it is possible the quantity of ab- 

 sorption may be very different from what prevails at the sur- 

 face. We also want a comparison of moonlight with sunshine. 

 The author states that these remarks are submitted to the Sec- 

 tion in hopes that some of its members may be induced to turn 

 their attention to supply these desiderata. It is also probable 

 that, if carefully examined, traces of jjolarization would be found 

 in that blueish grey illumination which has been noticed at the 

 same time. If they were wanting in the red part of the lunar 

 disc, this would decide the difference of their causes. This, 

 however, the author could not try, as, independent of other rea- 

 sons, the first were too faint to be visible through a plate of 

 tourmaline, the only analyser which he possessed. 



Account of some Observations made for the purpose of deter- 

 mining the Positions of the Axes of Optical Elasticity in ob- 

 lique prismatical Crystals. By Professor Miller. 



In crystals belonging to this system it is well known that 

 one of the axes of optical elasticity coincides with that crystal- 

 lographic axis which is perpendicular to the other two axes. 

 The object of the observations was to determine the position of 

 the remaining two axes of optical elasticity, and, if possible, to 

 discover some general law by which their position could be 

 made to depend upon the form of the crystal. After having 

 found the positions of the axes of elasticity in a variety of 

 oblique crystals, the hypothesis suggested by Neumann's ob- 

 servations on gypsum, viz., that the faces of the crystal could 

 be referred to the axes of elasticity as crystallographic axes, 

 was tried, but, except in one instance, did not succeed. In many 

 crystals no other relation between the form and axes of elasti- 



