^JourSJunTMrn NOTES AND MEMORANDA. 371 
published in the ' Journal of the Quekett Club ' (April), by Mr. John 
Hopkinson, F.E.M.S. The author gives a general account of the 
morphology of these fossil Hydrozoa, and tries to deduce from their 
history an argument against the theory of Natural Selection. 
Mr. Crooke's Binocular Spectrum-Microscope. — This instrument, 
which is made by Mr. Collins, and was exhibited at the soiree of the 
Royal Society, has, we are informed, been devised to obviate the dis- 
advantages of the ordinary spectrum-microscope. The principal 
features are the sub-stage and the box of prisms. The former carries 
a sliding-plate to hold the slit and apertures, a spring stop and screws 
for adjusting them, and a reversed object-glass. The slit and this 
object-glass are about two inches apart, and if reflected light is passed 
along the axis of the instrument, the object-glass forms a very small 
image of the slit in front of it. A milled head moves the whole sub- 
stage, and screws bring the image of the slit to any part of the field. 
Beneath the slit is an arrangement for holding an object of irregular 
surface or dense substance. The stage has a concentric movement, so 
as to permit the object to rotate, and enable the image of the slit to 
pass through it in any direction. The direct-vision prisms consist of 
three flint and two crown, fitted in a box screwed into the end of the 
microscope. By means of a pin they are thrown in or out of action. 
The object-glass screws on in front of the prism-box. By taking the 
illumination from the sky or a white cloud, Fraiinhofer's lines are 
visible, and by direct sunlight they are seen in great perfection ; the 
dispersion is sufficient to cause the spectrum to cover the whole field, 
and the achromatism of the lenses being nearly perfect, the lines from 
B to G are practically in the same focus. A double-image prism near 
the slit enables two spectra to be seen, oppositely polarized, and the 
variations in the absorption lines are at once visible. A Nicol's prism 
as polarizer, and another as analyzer, can be connected, and these 
enable the brilliant colours shown by some crystalline bodies, when 
seen by polarized light, to be examined. If a substance is dark- 
coloured, or the illumination not brilliant, the whole of the light 
should be passed up the tube to one eye ; but when the light is good, 
the appearance of the spectrum, and the power of grasping faint lines, 
are greatly improved by dividing the light with a Wenham prism, and 
using both eyes ; whilst the stereoscopic effect thereby communicated 
to some absorption and interference spectra, throws a new light on the 
phenomena. By using a spirit lamp instead of the illuminating lamp, 
the instrument answers admirably for examining flame spectra. The 
characteristic yellow, crimson, or green lines are seen beautifully 
sharp, on introducing sodium, lithium, or thallium into the flame. 
A Live-Box, which may be found useful by some of our readers, is 
described by Mr. J. W. Meacher in ' Science-Gossip ' for May. 
