42 Experiments on the Relative Intensities 



or horizontal sections of the cylinders is surrounded 

 by another circle, about three times as large, which 

 represents a flat horizontal circular plate of brass, 

 about iV of an inch thick, on which the cylinder 

 stands, and to which it is firmly fastened by solder. 

 These circular plates are placed on an oblong horizon- 

 tal plate of brass, through which the cylinders which 

 are continued below the circular plates pass in two holes 

 in the oblong plate which are made to receive them. 



To the lower ends of each of the cylinders which, 

 passing through the bottom of the wooden box which 

 constitutes the body of the photometer, project down- 

 ward, about an inch below it is fixed a thumb-piece or 

 handle (visible in the Fig. 2, Plate II.). 



These thumb-pieces serve for turning the cylinders 

 about their axes, which is done occasionally in order 

 to bring the shadows of the two cylinders which are 

 thrown on the field of the instrument to be of the 

 same width. The manner in which this is effected 

 will be evident, if we consider that, as long as the 

 vertical wing which is annexed to each of the cylinders 

 remains in the shadow of its cylinder, it cannot add to 

 the width of the shadow cast on the vertical plane 

 which constitutes the field of the photometer; but, as 

 soon as by turning the cylinder about its axis that 

 wino- is made to emerge from the shadow of the 

 cylinder on one side, the width of the shadow on the 

 field of the instrument will be increased. By these 

 means the widths of the two shadows which are com- 

 pared may at any time be made equal ; and they 

 should be so, in order that their intensities may be 

 compared with greater facility and accuracy. As often 

 as the two lights, which are the subjects of an ex- 



