NATURAL PHILOSOPHY. 135 



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lighted up with a red light, undoubtedly the effect of contrast. 

 T?ie thickness of the superposed mass was not enough to show a 

 greater effect than the almost complete absence of the red, and a 

 great diminution of the yellow. The ice was perfectly compact, 

 limpid, and with few air-bubbles. 



BREWSTER'S NEUTRAL POINT. 



In the April number of the " Philosophical Magazine," Sir David 

 Brewster says: "Dr. Rubenson has never been able to see, even 

 under the fine sky of Italy, the neutral point which I discovered 

 under the sun, and which, I believe, has never been seen by any 

 other observer than Mr. Babinet." 



The point in question can be easily seen in Philadelphia on any 

 clear day, when the sun is more than 20 above the horizon, and 

 I have reason to believe that it can be found with equal ease at 

 many other places in the United States, although I have not been 

 able to find any published observations except my own. Amer- 

 ican Journal of Science, Vol. 43, pp. Ill, 112. 



As all the phenomena of skylight polarization are very interest- 

 ing, and as some of its laws are still imperfectly understood, others 

 may, perhaps, be induced to turn their attention in this direction, 

 so as to determine whether the difficulty experienced by European 

 observers is owing to a higher latitude, to a moister atmosphere, 

 or to some other cause. 



A simple Savart polariscope is sufficient for making the obser- 

 vations. In positing Brewster's neutral point, I have usually 

 raised the lower sash of an attic window so that the bottom of the 

 sash will screen the sun from the polariscope. I have thus been 

 able, in every instance when the atmospheric conditions seemed 

 favorable, to see very distinctly the neutral point, and the 

 oppositely polarized bands above and below. PLINY E. CHASE, 

 in American Journal of Science, July, 1867. 



NEW POLARIZING PHOTOMETER. 



Mr. W. Crookes explained the construction and use of this in- 

 strument at the Dundee meeting of the British Association. Two 

 discs, emitting natural not polarized light, are placed in front 

 of it, and at a considerable distance behind is a doubly-refracting 

 prism of Iceland spar, rendered achromatic by a piece of glass, 

 which will separate the light emitted by the two discs into three ; 

 but, for the purposes of the instrument, the outer two must be 

 disregarded. The difference of intensity between the original 

 self-emitting discs is proportioned to the free polarized light found 

 in the central disc of light. This is again split up, and the differ- 

 ence of intensity between the first discs is ascertained from the 

 difference between these final ones. In comparing the light of 

 two stars, he makes use of Arago's polarimeter, which, twisted in 

 one direction, gradually cuts off one kind of light, and, when 

 twisted in the opposite direction, cuts off the other kind of light, 

 so that the intensity of the light is measured by the angle through 

 which the instrument must be turned. 



