On the Podura Scale. By E. M. Nelson. 395 



appeared in our Transactions in 1862.* He delivered it as his 

 opinion that the exclamation marks were upon one surface of 

 the scale, and that they were caused by the saw-edged nature 

 of longitudinal ribs. The next stage was the announcement, in 

 1869, by Dr. Pigott that the accepted images of the exclamation 

 marks were quite fallacious, and that they really consisted of rows 

 of beads like " peas in a pod." 



This started a long controversy ; some (Eeade, Maddox) agreed 

 with Pigott, others (Wenham) with Beck. 



Woodward at first sides ^vith Pigott, but subsequently adopts 

 Beck's view, while Mclntire does the reverse. 



Dr. Pigott describes the methods of illumination by which these 

 beads were produced. They were three in number, but it will be 

 sufficient for our purpose if we examine only one of them, 

 as the others were merely varieties of the same idea. Powell's 

 achromatic condenser had a cap with a minute hole in it for 

 centring purposes ; Pigott used this with the cap on ! By 

 this means an illuminating cone of the smallest dimensions is 

 produced, and bright diffraction spectra are formed at the peri- 

 phery of the objective. The objective is placed out of adjust- 

 ment, so that these peripheral rays are combined at a focus 

 differing from those at the centre, consequently the image of the 

 structure is a quadruple of the original ; for in the case of a Podura 

 scale the spectra at the periphery of the objectives they used would 

 be of the second order ; (four times 23,000 is 92,000, which is just 

 right for the water-immersions, which had then been recently intro- 

 duced, and which were from 1*1 to 1*2 N.A.). For the purpose of 

 placing the lens more completely out of adjustment, Pigott in- 

 serted in his Microscope tube an inner tube, containing two con- 

 verging lenses ; this he called an aplanatic searcher, but it was 

 merely a copy of the erecting tube in the Lister-Tulley Microscope.f 

 In 1872 Dr. Arnold dries the scales in a chemical oven, and 

 blows off the exclamation marks with an electric spark. In 1876 

 Dr. Woodward shows that false beading can be produced upon 

 gnats' scales by diffraction fringes thrown off from the ribs under 

 oblique illumination. 



In spite of this cogent argument, Pigott still maintains his bead 

 theory, and says % regarding the illumination : " General opinion 

 seems gradually to have come round in favour of pin-hole stops 



* T.M.S., X. (1862) pi. X. (pi. ix. is a misprint) fig. 1. This is an excellent 

 drawing by R. Beck, which should be compared with his later drawing (1865) in 

 " The Achromatic Microscope," pi. 7, fig. 1. Beck had then formed the opinion 

 that the exclamation marks were saw-edges on the longitudinal ribs, and he illus- 

 trated this idea in his figures. In the large figure on the frontispiece, amplified 

 1300 diameters, the exclamation marks run in straight rows of vertical lines, 

 which is hardly true to nature ; this figure should be compared with Pifiard's and 

 O'Donohoe's photomicrographs, J.R.M.S., 1893, p. 789, and 1906, p. 156. 



t See this Journal, 1900, p. 550. t M.M.J., xvi. (1876) p. 186. 



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