Page:Popular Science Monthly Volume 7.djvu/194

182 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 Journal of June, 1870, in which he repudiated such error, and described the beaded appearance as an illusion, 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 photographs 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 Microscopical 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 Review 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 aspects of the question; they are made from authentic scales, and are pronounced very perfect.



In further illustration of the difficulty of obtaining a true and reliable 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 interesting as showing the effect produced by the play of light upon a