276 
NOTES AND MEMORANDA. 
very exactly in the zero plane, which illuminates the crystal from the 
horizontal direction up to one of about 70°. By a mirror applied hori- 
zontally to the cube the crystal can he illuminated with the same slit by 
means of the reflected rays from the horizontal direction to one about 
70° downwards. In this way there will always be a luminous point 
reflected by the crystal along the axis of the microscope, provided 
that the face of the crystal makes with the stage an angle between 10° 
and 80°, and as it is sufficient to measure two of the angles a, h, c, and 
two of the angles a, /3, y, the measurement will always be possible, for 
if the face of the crystal should make with one of the faces of the 
cube an angle less than 10° or greater than 80°, this face will make 
with two other faces of the cube an angle comprised between 10° and 
80°. 
The Microscope applied to the Phenomena of Double Befraction . — 
In the same article M. Bertrand describes a method which he makes 
use of when greater sensibility is required than is obtained by the 
use of two crossed Nicols only. This consists of a thin plate made 
of four sections of quartz, alternately right and left handed, placed 
in the eye-piece of the microscope. This plate, of about 2^ mm. in 
thickness, gives between the crossed Nicols a field slightly bluish and 
uniformly illuminated ; but when a doubly refracting body is examined, 
the four sections present colours alternately blue and yellow, except 
in the position in which, by the ordinary method of observation, the 
field would be dark. In this latter case the sections remain uniformly 
illuminated, and of the same tint. 
This method is, on the whole, much more sensitive than that of 
simple extinction, inasmuch as two different colours, illuminated and 
side by side, have to be compared ; whilst when we have to estimate 
what is the position which gives the maximum extinction, two obscu- 
rities are compared, not by the side of each other, but one after the 
other. The same object can, however, be observed by the ordinary 
method of extinction and by that now described (using either the ordi- 
nary or the quartz eye-piece), and thus the results checked. 
The Causes of Buzzing in Insects. — M. J. Perez is the author of 
the following : — 
“ Since the experiments of Chabrier, Burmeister, Landois, &c., the 
buzzing of insects has been ascribed to the vibrations of the air 
rubbing against the edges of the stigmatic orifices of the thorax, 
under the action of the motor muscles of the wings. These latter 
organs play only a very small part in the phenomenon by modifying 
more or less the sound produced by the respiratory organs. 
I have repeated all the experiments of these authors : they have 
not always given me the results stated, or I have thought it possible 
to draw from them a conclusion different from theirs. 
1. It is quite true that by sticking together the wings of a fly 
{Sarcophaga carnaria) as Chabrier has done, we do not prevent the 
sound from being produced ; but it is not the fact that the wings can 
thus be kept completely motionless. The flexibility of these organs 
enables them at their base, where free, to obey the contractions of 
