Page:Popular Science Monthly Volume 89.djvu/924

 Popular Science Monthly

���Mounting of the seventy-two-inch reflector of the Dominion Observatory of Canada, to be installed at Victoria, British Columbia. The principles of construction are the same what- ever the size of the instrument, but the great weight of a large reflector makes the engineering problem a difficult one. The weight of the mirror cell with the mirror is six tons. The polar axis which is bolted to the pierheads, weighs ten tons. The skeleton tube weighs two tons. The dotted lines represent the path of the rays of light. The polar axis must be set parallel to the axis of rotation of tiic earth. In the latitude of Victoria, it makes an angle of more than

forty-eight degrees with the horizon

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