| Mourne, Secrosearss'] — Object-glasses and their Definition. 125 
destroying their transparency and rendering them brittle enough 
to be broken into fragments, 
The evidence, as it now stands, that the markings are elevations 
of the above figure,is as follows:—With the parabolic condenser 
(which approximates to an opaque illumination) the markings have 
the same characteristic form, and appear as material bodies of denser 
structure. Some years ago Mr. Ross showed me a Podura seale, 
mounted, uncovered, on a black disc, illuminated strictly as an 
opaque object, by a Leiberkuhn, which he had adapted to a }th of 
large aperture. The “note of admiration” markings were finely 
displayed, and appeared like detached bodies due to surface eleva- 
tions. This structure was afterwards confirmed by myself by 
spreading the scales on a highly-polished piece of Daguerreotype 
plate, and throwing a silver electro-deposit upon them. The de- 
tached metal film was then washed with a solution of caustic potash, 
in order to get rid of the scales, the matrices of which plainly 
showed the same surface form of markings. The late Richard Beck 
investigated this test as an opaque uncovered object with a 1th. 
The light was thrown on the scale by condensing lenses, and he 
gave illustrations of the appearances in his usual graphic style and 
published the following remarks :—‘ When the markings are at 
right angles to the direction of the light, they are illuminated on 
the sides farthest off. When they are in the same direction as the 
hight, with the narrow ends pointing to it, the broad ends appear 
like brilliant spots; but when the direction is reversed, 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 would 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.” 
In July last year I communicated to this Journal a principle of 
illumination by laying the objects on a glass surface, from which 
light was totally reflected, and using under such conditions the ordi- 
nary slides upon which objects are mounted. At all points of con- 
tact light will be admitted through, giving an analysis of the form 
of such parts, In the Podwra, as the light is thrown on more 
obliquely at an angle where total reflexion again begins to prevail, 
the scale appears marked over its surface with bright blue dots, 
