Microscopical Researches in High Power Definition. 277 



to spikelet, and is very black and distinct at the edge with, a first-class 

 glass, the objective aperture being reduced by using a condenser of 

 about 15° aperture. 



One of these terminators was placed between the spider-lines of a 

 delicate micrometer in such a way that the lined edge was apparently 

 half way between, and the interval on each side between them equal 

 to the thickness of the lined edge or terminating curve. Neglecting 

 the semi-thickness of the spider-lines, the distance between the latter 

 was read 15 millionths (15ttl) ; one-third of this is 5ttl, or the 200,000th 

 of an inch, whilst the general black margins of the ribs appear very 

 much less. 



As against transmitted light, causing diffraction, the extremities 

 of the spikelets were barely visible and did not appear one-third the 

 thickness of the black line edge or margin, or about the 600,000th of 

 an inch. Between the ribs forming the spikelets rows of about 

 four molecules could be seen stretching across like the rounds of a 

 ladder. The ribs were l-50,000th thick; the molecules about one- 

 third of this in diameter. 



On one occasion a scale was discovered " end " on. The ribs then 

 appeared rounded off and prominent, and of an intensely blue-black 

 colour ; projecting nearly the same amount in relief as they are 

 separated by the corrugated channels between them as semi-cylinders 

 (fig. 22) on one side, the other being quite smooth. 



The most surprisingly minute lines I have been able to detect were 

 seen in examining an extremely small species of gnat. Mounted 

 fresh and dry, the plumule hairs, about the 1-1 00,000th thick, were 

 themselves hairy ! — ciliated hairs. 



The antennas displayed central grooves also ciliated. And the 

 secretory organ, whence the plume hairs issued, formed a boss or cup, 

 around which a fringe of cilia of very minute magnitude was erected. 

 Comparing these with the familiar images of spider-lines reduced in 

 miniatures 140 times, I should not hesitate to estimate these objects at 

 a size approaching the millionth or microscopical unit (til). An 

 extraordinary instance of accidental magnification of these usually 

 invisible cilia with perfect distinctness, indeed, I might say, with the 

 most vivid definition (probably a power of 10,000 diameters) was dis- 

 covered to-day. See description of Plates, fig. 17. 



DESCRIPTION OF PLATES. 



Figures 1-2. Insect scales. Death's head moth under very low and high aperture. 

 Figure 3. Ordinary appearance of Degeeria domestica. 



Figure 4. Resolution of Podura domestica (same object as fig. 3) into dark and 

 light molecules, l-50th Powell and Lealand objective. 



Figure 5. The approved appearance of the spines of the Podura domestica (left 

 half). 



VOL. XXXI. X 



