170 



vision, at a power of 1600 diameters, the diameter of a 

 black spot magnified 1600 diameters, whose image presents 

 the visual angle of 2|' is found by the expression, 



~ X 1600 = siu 2i' = -0007272, 

 or d = -00000454; 



which is about the 200,000th part of an inch ; but it is 

 extremely doubtful whether this power can define so small a 

 quantity as this, therefore it is safe to assume that an object 

 presenting so small an angle as two minutes in the field of a 

 microscope cannot be defined clearly ; as to whether its out- 

 line be round or square, is totally beyond the powers of 

 human vision.^ 



But such minute particles may be capable of casting a 

 shadow, crescentic or otherwise, and a continuous line of 

 them, if sufficiently brilliant, would sensibly afiect the retina, 

 but without admitting ocular disintegration. Further, parallel 

 lines of such particles crossed by parallel lines slightly inclined 

 to the former, and nearly in the same place, give rise to 

 diffraction of a singular character. 



I viewed a set of parallel wires against the light, behind 

 which other lines, also parallel, could be seen ; rotating 

 gradually, the first set, suddenly at an angle of 15 degi-ees, the 

 latter appeared broken and no longer straight. (PI. X, Fig. 17.) 



Some difficulties of definition may be thus illustrated : — If 

 very fine lines cross each other at a general parallelism 

 amongst each other, the slightest deviation from equidistance 

 causes beautiful waves of interference, taking an infinite 

 number of forms. The more nearly the two sets become 

 parallel, the wider apart and broader appear the waves of 

 interference. Figs. 1, %. 



But if these sets of lines are composed of spheroid particles, 

 in contact and overlying each other, the difficulty of a distinct 

 separation is enormously increased, overlapping, then causing 

 the chief shadows. 



If the aperture of the object-glass be diminished to 50°, 

 two beads, exactly coinciding, transmit so little light as to 

 become dark points and a series of intersecting lines. The 

 dark parts form a black wedge-like marking, as seen in the 

 Podura. Figs. 15, 16. 



The dark lines or markings of interference become longer 

 as the two sets of lines cross at a smaller angle. Figs. 1, 3. 



When the lines intersect at any large angle, as J^5° and 



' Nobert.'s fanious lines ruled on glass 112,000 to the inch can only be 

 defined with Powell and Lealaud'a -J^p-th immersion lens. 



