442 
POPULAE SCIENCE EETIEW. 
these remarkable organs have not hitherto been observed, he has devoted 
much attention to them, and he thinks he is justified in expressing the 
opinion that they probably serve a very important and hitherto unsuspected 
purpose, in the respiratory system of the animal, and further, if he is right 
in his conjecture, that similar organs will probably be found to exist in 
many other insects. He thinks it possible, then, that these contractile 
sacs serve the purpose of pumps or syi’inges, by means of which air is drawn 
through the external orifices or spiracles, and propelled through the minute 
capillary vessels of the tracheal system. 
An Immersion Paraboloid is described by Mr. B. D. Jackson in the 
Journal of the Quekett Club ” for July. It is formed of a solid paraboloid 
of glass, ground to a different curve thnn the dry form, and instead of 
having its emergent surface hemispherically hollowed out, it is left nearly 
flat, a very slight concavity only being given. This concavity is so slight 
as to be hardly perceptible, but it is intended to permit the slide in contact 
with it, by means of the water film, to be moved to and fro vdthout danger 
of scratching the glass top of the illuminator — no very difiicult thing to do, 
in spite of the apparent hardness of the substance. The stop to prevent 
direct rays passing into the microscope is cemented to the lower surface of 
the paraboloid. The object {Eupodiscus argus) is shown by a quarter-inch 
binocular with a black field; the angle of the object-glass being about 
110°, a result he has not been able to attain so satisfactorily by any means 
previously employed. There is no loss of light by reflection from the lower 
surface of the glass, since the rays pass almost in straight lines from the 
curved sides to the focus. The ordinary test diatom slide, when mounted 
dry on the cover, as usual, presents a curious appearance, the field being dark, 
with a small spot of orange-brown light, occupying about one-fifth of the 
diameter, the spherules, however, being shown distinctly. He has not been 
able as yet to use this illuminator with higher powers, the fog surrounding 
the object unpleasantly. 
The French Erecting Prisjn is a Camera Lucida. — The July number of 
the ‘^Monthly Microscopical Journal ” contains a letter, in which the writer 
asserts most positively that this prism answers the purpose of a camera 
lucida. 
The Eegeeria Domestica, or Speckled Podura, is, according to Mr. Wen- 
ham Monthly Microscopical Journal,” July), when shown opaquely under 
a ^th or upwards, a specially beautiful object. The scales are apparently 
much thicker than in other species, and the ribbings or ! ! ! markings are of 
a reddish-brown colour — not beaded, but slightly constricted at regular in- 
tervals, like the short antennae of some insects, and in the deep intercostal 
spaces there are numerous thin septae, or transverse bars, very fine and dis- 
tinct, of a greyish tint. Both these and the slightly varicose ” spaces on 
the ribs may be displayed in the form of beads, by dodging the illumination. 
Where practicable, some form of opaque illumination should always be 
employed for verifying the structure of these objects, for we are in this case 
quite free from the errors of diffraction, which more or less accompany ob- 
jects seen by transmitted light, and cause an indistinctness of outline. 
The “ Wolf -rock ” under the Microscope. — Mr. S. Allport, in the Monthly 
Microscopical Journal,” says that, when examined by the eye or simple lens, 
