MKMOIR XVIII.] THE FEUUIC-OXALATE PHOTOMETER. 



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edge to edge opposite each other. We shall have 90 

 when they are rotated so as to be at right angles, and 

 180 when they are superposed with their edges coin- 

 ciding, or one of them be taken away. Thus, by setting 

 them in different angular positions, we can have all 

 quantities, from up to 180, and by removing them 

 entirely, reach 360. The lens will thus give an image 

 of a visible object always of the same size, its brilliancy 

 or intensity varying at pleasure in a known proportion. 

 In Fig. 42, A, B, B, D is a double convex lens set in a 

 wooden frame, F. Its face can be cov- 

 ered by two semicircles of blackened 

 card -board, one of which revolves on the 

 centre at c. In the figure they are rep- 

 resented as set at right angles, and the 

 quarter of the lens at A is uncovered. 



Fig. 42. 



To the foregoing description of the 

 chlor-hydrogen photometer I may add a 

 reference to another which I have very 

 advantageously used when extreme sen- 

 sitiveness was not required. It depends 

 on the employment of an aqueous solu- 

 tion of ferric oxalate. This substance, 

 which is of a golden-yellow color, may be kept for many 

 years without undergoing any change, if in total dark- 

 ness; but on exposure to a lamp or the daylight it 

 decomposes, carbonic acid escaping, and lemon -yellow 

 ferrous oxalate precipitating. If set in the sunshine, it 

 actually hisses through the escape of the gas. The ray 

 which chiefly affects it is the indigo, the same which 

 affects the chlor-hydrogen photometer and the silver 

 compounds used in photography. This ray, to produce 

 its effect, undergoes absorption, as might be anticipated 

 from what has been previously said in this Memoir, and 



