18 



by the other light — being afterwards counted off or plotted out for com- 

 parison. The beads thus shown might also be seen less distinctly by the reflex 

 illuminator and by the Wenham Paraboloid, and they were the heads of the 

 "tadpoles" in what was at present regarded as the true image of Podnra. He 

 would venture to designate this as the primary beading of the Podura Scale. 

 Then at a certain point in a very careful illumination, and under amplification 

 of the very finest character, there was a fine beaded structure to be seen in 

 the scale membrane itself, analogous to the beading seen in other scales. 

 This, he would venture to designate as the ultimate beading. It was, in 

 fact, a true beading of the scale membrane itself, while the other, or pri- 

 mary beading was, in reality, dae merely to a particular lighting up of the 

 tips of the featherlets, their hyaline shafts and the scale membrane being 

 left invisible, or almost entirely so. The true ultimate beading of the 

 scale membrane was closer than the primary beading laterally by about 

 two to one, while longitudinally it ran about five to one. In order to see the 

 ultimate beading by the paraboloid, it was necessary to light the scale so as 

 to make the featherlets invisible, and when these hyaline projections were 

 not seen except as a thin haze over the image of the scale membrane, the 

 true beading could be discerned. By transmitted light the images of the 

 featherlets and of the scale membrane were necessarily blended together 

 more or less completely, but with fine definition under an amplification of 

 about 2,000 diameters, and with exact illumination they could be demon- 

 stratively separated, if only the key given by means of this paraboloid 

 illuminator were fully realised. The definition of Podura featherlets on 

 black background was the best test he had yet seen for the most useful 

 qualities of a high power objective. The ultimate beading could be ap- 

 proached most easily through the study of the scale of the speckled Podura 

 — the structure being more easily resolved in that scale. 



Eeverting to the illuminator, by means of which he had been enabled thus 

 to unravel the structure of Podura scale, Dr. Edmunds proceeded to explain 

 the construction and action of the Wenham-Shadbolt paraboloid, illustrating 

 his remarks by drawings made upon the black-board, and showing wherein 

 his own paraboloid differed from previous appliances of the same kind. Dr. 

 Edmunds described his paraboloid as cut off at an exactly calculated dis- 

 tance below its focus, this distance varying in the four lenses which consti- 

 tute the set, and the plane top being made optically continuous, and as nearly 

 as possible optically homogenous with the substance of the slide, by means 

 of a cementing fluid of high refractive index, such as anhydrous glycerine, 

 castor-oil, copaiba-balsam, oil of cloves, or a new fluid — " plumboxyde of 

 glyceryl "—prepared by himself for the purpose. The latter fluid worked 

 exactly like pure glycerine, and could be made of much higher refractive 

 index. Chemically it might be described as glycerine with di-oxide of lead 

 introduced into its molecular structure, in place of its equivalent of hydroxyl, 

 but as he had not completed his work on this refracting medium, he would 

 not now refer to it further. These paraboloid lenses acted upon the prin- 

 ciple of total internal reflection, and each one was calculated for the thick- 

 ness of the slide beneath which it was to be used, so as to converge upon 



