l82 



THE POPULAR SCIENCE MONTHLY. 



him, now^ that in the best object-glasses small residuary aberration 

 existed. 



This slur upon the best object-glasses brought out Mr. Wenham 

 with a paper in the Microscopical JoxLrnal of June, 1870, in which he 

 repudiated such error, and described the beaded appearance as an illu- 

 sion, obtained by a trick of illumination, and by examining the scale 

 with the microscope out of focus. 



At the June meeting of the Royal Microscopical Society, a letter 

 was read from Colonel Woodward, of Washington, inclosing photo- 

 graphs of the podura-scale, showing what he considered to be the 

 true appearance. These photographs showed the spines. Colonel 

 Woodward, however, reserved his opinion, and asked for a specimen 

 of the true test podura-scale. 



Dr. Maddox, in August, exhibited various photographs of podura- 

 scales, which Mr. Wenham commented on in a paper to the Micro- 

 scopical Journal of September following, which merely reiterated his 

 views that the ''spines'''' were the true appearance of podura-scales. 



The Rev. J. B. Read, in the Popular Science Revleio of April, 1870, 

 appears to accept Dr. Piggott's views entirely, and writes : " I can 

 now see with my own powers what has been before invisible, viz., the 

 beautiful beaded structure of the whole test-scale, as discovered by 

 Dr. Piggott." 



It would be tedious to continue the subject and give even an out- 

 line of the papers and discussions that have been provoked by this 

 knotty question: I shall, therefore, conclude by stating that Colonel 

 Woodward has since produced two photographs, showing the two as- 

 pects of the question ; they are made from authentic scales, and are 

 pronounced very perfect. 



Fig. C. 



In further illustration of the difficulty of obtaining a true and re- 

 liable image of an object when viewed under the microscope with high 

 powers, I offer drawings which have been made by Mr. Ralph H, 

 Westropp, B. A., T. C. D., of Allyflin Park, England, and represented 

 at Figs. 4, 5, 6. These figures all represent the same object, a scale 

 of podura viewed under different phases of oblique light ; they are in- 

 teresting as showing the effect produced by the play of light upon a 



