THE THEORY OF APERTURE IN THE MICROSCOPE, 333 
ame a eae ite ee el oe i ae ee 
differences of composition of the particles composing the object ; 
so that nothing more can safely be inferred from the image as pre- 
sented to the eye than the presence in the object of such structural 
peculiarities as will produce the particular diffraction phenomena 
on which the image depends.” 
In examining such minute structure as the strie of a diatom, it 
has been shown* that, if we take the distance of the striz as 6p 
(about zy355 Of an inch) and the wave-length of light as .55,, in 
the centre of the luminous spectrum, the first diffracted ray will be 
directed outwards at an angle of 66°. 5 from the axis, when central 
light is used, and this spectral ray will be collected on each side 
of the axis by a dry objective of 133° angular aperture, as in Fig. 
43. In balsam the same rays will be deflected at an angle of 
372°, and will therefore be received by an objective of 75° balsam- 
angle. If the distance of the stris be twice the former quantity, 
or 1.24, then the dry objective of 133° air-angle will admit two 
diffraction rays on each side the axis, while an immersion lens 
of 133° balsam-angle will admit three such rays on each side. 
This is another proof that an angular aperture in air is not an 
optical equivalent of the same angle in balsam or oil, but that the 
latter is something more, and its capacity for showing things as 
they are, when the structural particles under investigation bear com- 
parison in minuteness with the wave-lengths of light, must be 
greater also. 
As the structural elements become larger, the diffraction rays 
become contracted within smaller angles, until medium and low 
angles are capable of transmitting them, and the object at length 
becomes imaged upon common dioptrical principles. 
If, instead of using central illumination, as in the last case, we 
now pass an od/ique pencil of light through the smaller diatom, the 
direct ray and one spectral ray will be just received by an objective 
of half the angle of the former, or 66°5 in air, as in Fig. 44; and 
this aperture will be the 
theoretical minimum cap- 
able of separating lines = 
.6 w. If we want to see 
images more characteristic 
of the real structural ele- 
ments than mere lines, we 
must use wider apertures. Fig. 44. Fig. 43. 
We may also diminish the wave-length of the light by employing 
rays corresponding with a part of the solar spectrum nearer the 
blue, and thereby gain the advantage of a slight decrease in the 
angle of diffraction. If the line E of the solar spectrum be taken, 

* See Journal R. M. S., 1881, p. 357. 
