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light reaching this latter till after the two reflections, by which it is 
confined to one side of the base of the paraboloid and of course 
reaches the focus and the object at one side only, and by rotating 
the paraboloid (I believe it ought to be an ellipsoid) and prism to- 
together, this oblique illumination may be carried round the entire 
field of the microscope. 
‘This arrangement has realized my expectations. Having got 
it ready in October last, the first object on which I tried its capa- 
bilities was the Podura scale, an object which has been observed 
vastly more than any other test. I immediately saw appearances in- 
dicative of a structure entirely unknown before. In one azimuth of 
the light the scale appeared obscure and structureless, except that 
it was studded over with minute, nearly transparent dots, more re- 
sembling oil-glands in the leaves of the myrtacez than anything 
else with which I am acquainted. 
“ By rotating the light, faint shadows began to be visible in con- 
nexion with these dots, and when one-fourth of a revolution or 
thereabouts was completed, the dots had disappeared and the scale 
seemed to be covered, thatched as it were, with short, slender, cy- 
lindrical appendages; continuing the rotation in the same direction 
the scale gradually became obscure as before; these appendages 
ceased to be visible, and at the end of a second quarter revolution 
the luminous dots again appeared, more faint than at the opposite azi- 
muth, still unquestionably there; completing the revolution, the 
same succession of appearances recurred until, on returning to the 
original azimuth, the dots re-appeared as luminous as at first. 
“From frequent repetition of this observation, I am satisfied 
that this scale, instead of being covered with sculptured lines or 
folds, as was once believed, or with dark hairs, as figured by Queckett, 
is in reality covered with cylindrical appendages like quills, but 
which are either hollow or quite translucent through their axes. 
“T infer this from finding that the several appearances are inva- 
riably and only seen with certain azimuths of the illumination; the 
bright dots when the light enters from the base of the scale; the 
fainter ones when it enters from the opposite extremity; and the cy- 
lindrical appearances when it enters at the side of the scale, in the 
longitudinal direction of which these appendages lie. 
“ Again, with the scale of the Lepisma, this is clearly seen to be 
VOL. V. 21 
