MEMOIK XVIH.] THE SELECTIVE PHOTOMETER. 



269 



Fig. 43. 



Let a wooden box, A B (Fig. 43), six inches long, two 

 wide, and two deep, with perforations at A and B in its 

 ends, be provided; in the centre of its top an aperture 

 three quarters of an inch in diameter is to be made. 

 The box must be blackened interiorly, and a rectangular 

 prism of wood, C, be placed in it, with its right angle in 

 such a position that its edge bisects as a diameter the 

 circular aperture ; over this wooden prism a piece of 

 white paper is pasted, care being taken that where it 

 bends over the right angle of the prism it is folded 

 sharp. So far the reader will recognize in this Ritchie's 

 photometer. Upon the aperture in the top of the box 

 a glass trough, g 7^, is placed ; it is made by cutting a 

 circular hole an inch in diameter in a piece of plate-glass 

 one third of an inch thick, and laying on each side of it 

 a thin piece of plate-glass. This forms a circular trough, 

 in which a strong solution of sulphate of copper and 

 ammonia may be enclosed. Over the trough a tube, d, 

 eight or ten inches long, is placed so that the eye may 

 see distinctly through the aperture in the top of the box 

 the disk of paper, and more especially its dividing di- 

 ameter. 



