IOWA ACADEMY OF SCIENCES. 181 



A RULING ENGINE FOR MAKING ZONE PLATES. 



BY W. M. BOEHM. 



About a century ago the corpuscular theory of light was 

 falling into disfavor among scientific men in general and 

 the wave theory was taking its place. Among the many 

 contributions to the subject we find the writings of Fresnel. 

 It was he especially who struck the decisive blow at the 

 dying hypothesis. That part of his work which is of par- 

 ticular interest to us is his application of Huyghen's prin- 

 ciple to phenomena of diffraction. 



According to this principle, every particle on the wave- 

 front, IB, figure 19, may be regarded as the source and 

 center of a new wave. Suppose our figure were a section 

 through a wave. A is the center of the disturbance; IB 

 part of the wave-front, and F some particle on the straight 

 line passing through A B, and farther from A than the wave- 

 front. We know that, in a homogeneous medium, IB, the 

 section through the wave-front, will be an arc of a circle. If 

 there is no opaque obstacle on the line between A and F, the 

 wave will in time reach F, for light travels in straight lines 

 in such a medium. But, suppose we had an obstacle 

 between A and F. Why is it that we will not receive light 

 from the other particles on the wave-front if, as we have 

 said, each particle there acts as the center of a new wave? 

 In other words, why will light not travel in a crooked or 

 curved path in a homogeneous medium? Fresnel's expla- 

 nation is not the most satisfactory but is sufficient for the 

 present purpose. 



He divides the wave-front into a number of zones. In 

 the figure these are shown as arcs of a circle. The line 

 IF is equal to B F plus one half a wave-length. The next 



