274 



Dr. G. W. Royston-Pigott. 



the objects in view under powers varying from 1,000 to 3,000 

 diameters, by the aid of the camera lncida. 



Scales seen in profile " end on. 11 



Figs, a, h, c, represent the fortnnate vertical appearance of a scale 

 of the Petrobius maritimus, a Podura freqnenting the coast limestone 

 rocks. The scale was luckily canght between the slide and cover 

 during the process of remounting (a) . A slightly deeper focus showed 

 appearance (b), deeper (c) ; a little more pressure caused the scale to 

 turn over and lie nearly flat, recovering its vertical position on the 

 removal of the pressure. This phenomenon solved the problem once 

 for all, that the under side is smooth, the upper ridged. Black beads 

 could be detected in (c) at the tips of the ridges. Having secured this 

 fact, attention was next turned to examination of the tips of the 

 spikes with a very exquisite definition of 2,000 diameters. On search- 

 ing many specimens, a full-grown scale was found and accurately 

 drawn from the microscope by an assistant. Never was a search 

 better rewarded, for not only were the great ribs found beaded on 

 both sides, but up to the very tip (fig. 12d) ; while transversely three 

 or four spherules stretched across, in short rows, apparently supple- 

 mented by another parallel set placed in contact in a lower focal plane. 

 Increased pressure being applied the scale was ruptured ; the scale oil 

 then flowed out, but returned, insinuating itself along some of its old 

 channels ; but while pressed out, the whole molecular structure became 

 visible in the obliterated part; attempted to be shown in the en- 

 graving. Besides these a great number appeared to have burst forth 

 from the disruption. 



Podura macrotoma. — This little dark-brown leaping insect is very 

 commonly found in the early spring under old timber and brick-bats. 



A young scale is represented faithfully in fig. 14, a full grown one 

 in fig. 15, and the cross beading under a power of 3,000 in fig. 16, the 

 ribs being quite out of focus in an upper focal plane. In fig. 15 several 

 spherules have escaped and lie irregularly upon the surface and 

 outside the spikelets, each showing a bright focal point and annulus. 



It should be here stated that an object-glass was used as condenser 

 of only 15° angular aperture. Large aperture renders the spherules 

 faint and almost invisible. Direct light was employed and lard oil 

 with a Seibert l-32nd, or No. XI objective of excellent quality. Great 

 niceties were also attended to, such as — 



(1.) Varying the length of the observing tube between 5 and 16 

 inches, changing the eye-pieces and especially altering the aberrations 

 by the screw collar. 



(2.) "Varying the immersion fluid. 



(3.) Varying the illumination by means of a series of objectives as 

 condensers having angular apertures between 10° and 85°. 



