Microscopical Researches in High Power Definition. 267 



Example 2. — A microscope with ten degrees aperture considerably 

 diminishes the relative thickness of the black annnlus. 



Example 3. — With forty degrees it is much attenuated and it 

 vanishes altogether when the angular aperture reaches a certain rela- 

 tion to the lenticular refraction. 



The vanishing limits vary with the nature of the refracting 

 spherule, and also with the angular observing aperture and the direction 

 of the axis of illumination.* 



If it be formed of plate glass whose index of refraction into air is 

 1*500, the vanishing angular aperture is 83° 36'. But as the index is 

 higher, this angle increases with heavy flint glass (m=l"988) to 

 164° 6'. In both these cases the black defining annulus of the 

 spherule is in these limits attenuated almost to evanescence. 



Glasses, therefore, of small angular aperture develope the broadest 

 black outlines in a minute refracting molecule. 



The change of appearance of translucent bodies composed of masses 

 of refracting molecules is very finely shown in observing a variety of 

 scales forming the dust of moths and butterflies. 



Example 4. — Featherlets of the death's head moth. Low angular 

 aperture, 10°. The whole animal bristles with black feathers, armed 

 with three or four long black spines : all of a dark but rich umber 

 colour, tipped intensely black ; each spine shows an exceedingly thin 

 line of light running centrally up between two broad intensely black 

 margins. (Exactly what is seen when examining a thread of spun 

 glass with low aperture.) (Fig. 1, Plate 3.) 



Increasing Aperture. — Colours pale. Light flashes through. The 

 spherules begin to appear edged with black annuli, which gradually 

 attenuate with higher angled glasses. (Direct light.) (Pig. 2.) 



(^g. ^ro 



It is remarkable how the colour changes as the aperture increases, 

 through paler shades, until a general sparkling radiance appears to 

 steal through the mass of molecules formerly darkling with the uni- 

 versal presence of black annuli due to low aperture. The black 

 edgings also of terminating membranes extending from spikelet to 

 spikelet, and those of refracting tubules become indistinct. 



The obliteration of marginal shadows is well shown by first using 

 low and then large aperture on another very beautiful object, viz. : — 



Example 5. — Plumelets of the HipparcJms Janira. Aperture 20°. 

 Power 200. The filaments of the plume and their clubbed ends are 

 all intensely black (Plate 4, fig. 13<x). 



* " The use of a new Aberraineter, for testing Aberration and the Effect oi 

 Aperture," "Quart. Jour. Mic. Science/' January, 1871. The angles subtended by 

 the black shadow are there calculated and tabulated. 



