62 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



The image of the particular wave lenccth under investigation was 

 reduced to 8x0.23 inm and focused on the metal under examina- 

 tion. The metal in the form of a wedge, semi-transparent at one 

 end and opaque at the other, was deposited by distillation from 

 an incandescent wire in vacuo. The techniciue of obtaining the 

 metal in the form of a wedge was described in a recent article 'oy 

 the writer.^ The method consists essentially of a wire heated 

 to incandescence by means of an electric current, while a quartz 

 plate upon which the wedge is to be deposited is placed below 

 and to one side of it. If the whole is now placed in vacuo, 

 the metal will vaporize and condense upon the quartz surface. 

 Since there is no reflection of metal from the walls of the in- 

 closure it can be shown that under these circumstances a section 

 of the surface thus developed upon the quartz plate is of the form 



8a ^ 

 y = a where 2a is the height of the wire above the 



plane, y being the thickness of the depovsit at any distance x 

 out from the perpendicular dropped from the wire to the plane 

 containing the plate. The section of the metal thus deposited, 

 forms a surface at a distance of four centimeters from the wire 

 (2a^=2cm.), which approaches a right triangular wedge with 

 a variation of five per cent at either end. 



This wedge (1X2. 5cm) was placed, metal side facing the 

 light, along the axis of a Faraday Cylinder, having slits cut in its 

 opposite sides to allow for the unobstructed passage of the light. 

 This cylinder was electrostatically shielded and connected to an 

 electrometer. The wedge was clamped at its thick end to an 

 indicator which slid over an engraved scale. The rod carrying 

 the indicator and the wedge could be moved so that successively 

 thicker layers of the metal could be photoelectrically examined 

 as it passed in front of the illumination. The wedge was thus 

 exposed at millimeter intervals through its entire length with 

 the result that successive increases in thickness of the metal 

 exposed, gave the requii-ed in photoelectric current desired. 



Plotting the resulting photoelectric current against thickness 

 of the metal thus exposed, developed an exponential curve, 

 which saturated at a point where further increase in thick- 

 ness produced no further decrease in current. This gave a true 



igituhlman. Journral of tlip Optical Society, vol. ], p. 78, 1917. On 

 the Preparation of Metallic Mirrors, Semi-transparent and Transparent Me- 

 tallic Films and Prisms bv Distillation. 



