^jZraai,TuTyT£T of Biatoms and Podura Scales, 29 
by their close adhesion prevent the total reflexion of light at the 
S]pot beneath, consequently it is like a hole made in a dark lantern, 
and a flood of light escapes through the scale. Herein, as with the 
parabolic condenser, the markings are the most intensely illumi- 
nated, and appear the brightest ; and under a good yV or con- 
trasted with the surrounding darkness, the aspect of the Podura is 
at the first glance so novel, and different to what we have been 
accustomed to, that the black interspaces may be mistaken for the 
markings, jpointed in the reverse direction ; but on looking again, 
there are still the characteristic note of exclamation " figures, the 
same as ever. The rays from the lamp must first be rendered 
parallel by a bull's-eye condenser. On now sliding this on the 
table sideways, the light will gradually vanish from the object, and 
give a dissolving view ; and at last there is only the faint outline of 
the scale, with its barely perceptible blue surface dotted over irregu- 
larly with minute bright blue circular spots. This appearance is 
so different from anything before seen in the Podura, that were I 
to exhibit it as such, not one of its numerous friends would recog- 
nize it. But on bringing the light gradually forward again, we 
have at once most palpable proof that there is no deception. These 
spots of light emanate from the butt-ends of the markings; for 
these, having both a higher refractive power, and being also, perhaps 
from their prominence, in more intimate contact with the glass, are 
the last portions to admit the light, at the time when the angle 
becomes so great that the margin is approached, when total reflexion 
again begins to prevail in the glass against the less refractive power 
of the scale, which causes its transmission. 
As the light is again drawn forward, it plays over the individual 
markings, and gradually discloses them, from the first bright point to 
the taper ends. As in this mode of illumination we see the scale as 
nearly as possible as an opacjue object, without the false appearances 
arising from refraction, I think that it affords some proof that the 
markings of the Podura consist, in reality, of the taper bodies 
generally known, and which have considerable refractive power and 
some amount of opacity, and that there are only one series of them. 
But instead of being planted on one side of the scale they are 
enclosed between two membranes. If they were set on one side of 
a single membrane some difference would be observable above and 
below, and it is well known that there is none. And further, the 
unprotected markings could be displaced or detached, if they lay on 
the outer surface only ; and, finally, the mode of illumination herein 
noticed would merely allow particles of the light to find their way 
only through the restmg-points of the markings, instead of over the 
whole area of the scale. This affords strong proof that there is a 
flat membrane in close contact and adhering to the glass surface, 
destroying the total reflexion in the entire space occupied by the scale. 
