270 



Dr. G. W. Royston-Pigott. 



glass, a very fine l-8th is used, at the same time that very oblique 

 light illuminates the object, by tilting the axis of the condenser, black 

 crescentic annuli are clearly developed (fig. 6). There is no doubt 

 the view manipulated is entirely due to shadow thus developed and 

 the preservation of marginal darkness. In this way the object may 

 be shown with a very fine half-inch objective. The molecules are 

 indeed huge compared with other visibilities : perhaps the explanation 

 here given may enable other observers to confirm their existence in 

 this comparatively easy object. 



The magnificent Oil Immersion Lens, by Zeiss, of Jena, utterly fails 

 with its full aperture to show the appearances just described. It 

 displays grandly indeed the long corrugations, tubules, or ribs cross- 

 ing each other irregularly, but entirely misses their beautiful contents : 

 fully charged though they be with spherules (see fig. 7, Plate 3). 



The total disappearance of this interesting structure, on the field of 

 such a glass, is both surprising and instructive to the beholder. 



Experiment 7. — Data of Resolution. Inferior l-8th P. and L. 140° 

 Objective, 1862. Aplanatic condenser 44° of aperture (Wray half- 

 inch). February 5th, 1879. Power 800: cloudy daylight. Direct 

 light. 



The ribs are interrupted and alternately mottled blue and rose 

 colour, slightly pink-coloured molecules without shadow annuli just 

 discoverable peeping between the interrupted ribs. (The lowest focal 

 plane must here be diligently searched.) 



Example 8. — Data. Objective by Powell and Lealand of exceed- 

 ingly fine quality. (Screw collar has 2-J turns and was opened to 25° 

 from zero.) Cloudy daylight ; Same illumination. 



Result. — Eidolic black dots very much smaller than spherules dis- 

 cernible between very decisive appearance of spines or exclamation 

 markings (!!!,)• At a very high plane of focal vision, spines alone, 

 the familiar optical test (fig. 8). This singular result requires investi- 

 gation. Eidolic black dots are generally discoverable on a focal plane 

 above the centre of refracting molecules. 



Example 9. — Newest form of P. and L. l-10th immersion, a glass 

 of remarkable precision. 



Lowest attainable focal plane, with "collar" fully open, so that 

 "nose " is in contact with the " cover." Daylight as before. Aper- 

 ture of direct illuminating cone 44°, formed by Wray half -inch. 



Result. — The spines are broken into double rows of minute black 

 dots. Shadowy white beads are glimpsed (with most scrupulous 

 attention to focal and collar adjustments) between the spines. But 

 when the condenser is tilted, strong dark crescentic annuli suddenly 

 appear on the shadowy whitish molecules. 



Example 10. — Data. 3 p.m. Sun dimly visible : Powell and Lea- 

 land's diaphragm No. 2, diameter 0'2 to reduce angular aperture of 



