268 



Dr. G. W. Royston-Pigott. 



Aperture 44°. — All still intensely sharp and black with the fine 

 definition of a Wray " half- inch." 



Aperture 55°. — Wray glass. Margins thinner ; the filaments begin 

 to be translucent and at the spherically clubbed ends a focal point of 

 light appears. 



Aperture 94°. — A remarkably fine l-6th by Beneche of Berlin. 

 Power 600. Increase of translucency at every part ; black margins 

 attenuated. Club margin much thinner (fig. 13b). 



Aperture 140°. — Fine l-8th. Power 800. Black margins almost 

 attenuated to invisibility ; clubs translucent altogether ; no annulus. 



It is interesting to state that the thickness of the filaments of this 

 plume vary between -g- oiroo" an ^- 90M0) J e ^ these are beautifully distinct 

 (aperture 12° and 50 diameters 2 -inch objective), and yet under this 

 amplitude a single filament subtends* less than 20 seconds of arc. 



The exceedingly black and sharp appearance of these filaments 

 doubtless accounts for their actual visibility under this very small 

 visual angle. Now this exactly represents a line 1-1, 600,000th 

 thick (considerably less than a millionth of an inch) seen under a 

 power of 1,000 diameters. f 



If then the minute fibriUse of the plume can be clearly distinguished 

 as closely packed black lines, at a visual angle of twenty seconds, 

 with low aperture of twelve degrees, this result is fatally opposed to 

 the popular idea that very close lines, or very minute lines or bodies, 

 can only be distinguished with large angular aperture. These lines 

 were most sharply seen, though less than l-80,000th thick (fig. 13<z, 

 Plate 4). 



But besides the black sharply defined ring or annulus always de- , 

 veloped in a refracting molecule by using low aperture, oblique illu- 

 mination produces a thorough change in this black ring : it is trans- 

 formed into various black crescents ; and a row of such molecules 

 approximately appears as a continuous black line sometimes notched. 

 A tubule also produces a variety of marginal shadows according to 

 the angular aperture of observation and according to the arrangement 

 of internal molecules, and their combined shadows produce a variety of 

 effects of an important character (fig. 6). 



It follows from these considerations that researches upon inex- 

 hausted structures (those inadequately resolved for instance), must be 

 conducted with especial reference to the development and detection 

 of shadow annuli or bands, or notched black lines, and with special 



# 9 _ arc x power = v 1 . = 13 „ 



rad. 10 inches 16000 ' 



j n arc l-r-1600000 , AAn 1 

 power = x 1000 = 



rad. 10 16000 



