NATURAL PHILOSOPHY. 125 



from the polarizer without being really polarized. He remedied 

 this by intercepting the liglit with a flint concave lens before it 

 reached the polarizer, so that the whole mass of rays, being pi-o- 

 jected in a parallel direction, was completely polai-ized. On leav- 

 ing the polarizer, the rays were again converged, before passing 

 through tlie crystal, or other object to be exhibited, by a small 

 achromatic lens, which thus acted as an achromatic condenser. 

 It was stated that this arrangement effected a most imi^ortant 

 increase in the brilliancy of the object exhibited on the screen. — 

 J. Traill Taylor, in Reader. 



COMPARATIVE INTENSITY OF THE LIGHT OF THE MOON 

 AND OF VENUS. 



On June 20, 1865, at 3 A. M., the moon and Venus were in 

 conjunction, in the latitude of Lyons, France, so that both bodies 

 could be seen in the same field of vision. This afforded an oppor- 

 tunity of comparing the light received from them. The surfaces 

 taken for comparison were those affording rays at the same angle 

 of incidence ; and, on the moon, the region was that between the 

 craters Rocca and Eirchstadt, over the very brilliant surface to the 

 southeast of Grimaldi. It was found that the light from this 

 brightest part of the moon was only one-tenth of that reflected by 

 the surface of Venus. — Chacornac, in Comptes Beiidus, 58. 



TRANSPARENCY OF THE SEA. 



Father Secchi has come to the following results, from experi- 

 ments made near Civita Vecchia, at from six to twelve miles from 

 the coast, the sea being clear and calm. It was found that the 

 maximum depth at which a white disk, ten feet in diameter, was 

 visible from the surface, when the sun was sixty degrees above 

 the horizon and the sky clear, was about one hundred and forty 

 feet. In descending, white disks appeared first of a light green 

 color, next of a clear blue, then the blue became gradually' darker, 

 until, at the depth mentioned, they could not be distinguished. 

 Yellow or sand-colored disks, ceased to be visible much sooner 

 than white disks, becoming invisible at depths varying from fifty- 

 five to eighty feet, according to their tint. 



CURIOUS EXPERIMENT. 



The following good lecture experiment has been suggested by 

 M. J. Niekl6s. With the following pigments, he paints a spec- 

 trum, which shows all the colors, either by gas or candle-light ; but 

 shows only black and white, with a soda flame (alcohol and salt) . 



Color by daylight. Pigment. Color by soda flame. 



Red, Ochre, Black. 



Orange, .... Biniodide of mercury, > TVVi'fa 



Yellow, .... Chromate of lead, 5 * * "^'*^'^®' 



Green, .... Manganate of baryta, > -ni .i. 



Blue, Aniline blue, ^ * * ■^^^°'^- 



11* 



