182 IOWA ACADEMY OF SCIENCES. 



line is half a wave-length longer than I F, etc., every line- 

 is half a wave-length longer than the preceding. We will 

 not go farther into these details since they are now too 

 well known. Suffice it to say that, by simple mathematical 

 means we can show that the motions given to F by any 

 half zone are exactly balanced by the motions from the 

 succeeding half zone as a result of destructive interference. 

 There is one exception to this, i. e., the motion transmitted 

 by the particles in the immediate vicinity of B. The 

 motion from these particles is what causes the motion 

 of F. It is evident that when we interpose some obstacle,, 

 like the dark film of a photographic negative, so that the 

 motions from each alternate zone cannot arrive at F, we 

 will have the motions from the remaining alternate zones 

 arriving a F, practically, at the same time and in the same- 

 phase. Anegativeof this kind will have a series of concentric 

 circles whose i^adii are proportional to the square roots of 

 the respective numbers of the circles. Soret, in 1875, pro- 

 duced a plate of ninety-eight dark circles and obtained 

 some very satisfactory results. In 1898 Prof. Wood, then 

 of the University of Wisconsin now of Johns Hopkins,, 

 made a plate of 115 dark circles. He also produced phase- 

 reversal plates, /. e., plates which were entirely transparent 

 but in which the alternate zones would retard the motion 

 by one half a wave length. His results, in addition to a 

 plate, were published in the Fhilosophical Magazine for 

 that year. His plates were made by first producing a 

 drawing and photographing it afterwards. He used an 

 ordinary beam-compass. Now, anyone who has had the 

 fortune to manipulate one of these instruments can appre- 

 ciate the value of a steady hand, when he obtains good 

 results. It was with the intention of producing these 

 plates with greater ease and accuracy that an engine, or 

 rather machine, was built at the University of Iowa this 

 year. 



Mechanical details are always tiresome but, if you will 

 draw on your liberal supply of patience, we will consider 

 the engine which has enabled us to make plates of, not 230 



