86 
Beck^ on the Scales of Lepidocyrtus. 
In the present instance the most conclusive proof of the 
perfectly transparent nature of the whole substance of the 
scales is^ that the markings almost entirely vanish when the 
irregularities of the surface are removed by the presence of 
moisture. Fig. 3 represents this appearance on the Podura- 
scale ; and I cannot but think that Dr. Carpenter is mistaken 
when he says that such scales have lost a portion of their 
superficial layer by some accidental injury.^^ 
That the particular wedge-shaped bodies of the Podura- 
scale are elevations upon the surface can^ I believe^ be satis- 
factorily proved by viewing the scales under illumination from 
above. This method would generally be spoken of as making 
them opaque objects; an expression^ however^ which cannot 
be correct_, for unless the real nature of an object be changed, 
if it be a transparent object it must remain so still, whether 
it be illuminated from above or below. If, in the latter case 
(that is with light from above) these scales be placed under 
a one-eighth and third eye-piece, without any coveys (for with 
the oblique illumination that is necessary under such a power 
the thin glass is a most perfect reflector), quite sufficient light 
can be obtained by means of the usual large and small con- 
densing lenses combined, and we have the following result : 
When the markings are at right angles to the direction of 
the light (fig. 7), the side farthest ofi" is illuminated; when 
they lie in the same direction as the light, with their narrow 
ends pointing to it (fig. 8), the broad ends appear like brilliant 
spots, but when this direction is reversed (fig. 9), the light 
from the points is so slight that the scales appear to have 
lost their markings altogether. Now, if the object were an 
opaque substance, this result Avould have been a convincing 
proof that the markings were depressions ; but as we know it 
to be transparent, it follows that these particular appearances 
can only be produced by elevations. 
At the present time the next and most natural question 
will be, how do the scales appear under the binocular micro- 
scope? Here the same caution is necessary, from the object 
being transparent. After witnessing some very curious effects 
arising from this cause when the object was under dark field 
illumination, I found, entirely to my own satisfaction, that 
by selecting a peculiar scale in which the markings had a 
curious turn in their direction and were very distinct, and by 
using a condenser-illumination from above, on both sides, 
that a series of toothed ridges, the profile of which might be 
said to resemble the edge of a saw, might be brought into 
view ; and my own belief is, that the markings upon this and 
all other varieties of Podura-scales are more or less eleva- 
