the many important phenomena which may be explained by the study of pedesis, and I propose to follow up the investigation of this movement in regard to the several substances which tend to increase it."

The Preparation of Thin Sections of Objects of different Consistency. -In investigating the anatomy of corals Mr. G. von Koch, of Darmstadt found the calcareous skeleton one of the greatest drawbacks to his researches, as it often prevented any observations of the structure of these animals. Sections of decalcified pieces give good results only in special cases. Generally the decalcifying and still more the succeeding operations so disarrange the separate parts that their original relative positions can scarcely be recognized, and the structure of the calcareous parts is of course entirely lost. He applied the following method to overcome the difficulties, and obtained preparations that would show very clearly the structure, form, and position of the different elements. Pieces as small as possible of the object to be cut are stained thoroughly, and after rinsing, all water is got rid of by means of weak and afterwards proof alcohol. The pieces are then put in a cup filled with a very thin solution of copal in chloroform. (This is easily made as follows:-Pound the coarse pieces of broken copal in a mortar with fine sand, pour chloroform over the fine powder thus obtained, and filter the solution.) Then slowly evaporate the copal solution by putting the cup on a piece of pottery ware, which is warmed by a common night-light. The slower the evaporation the better. When the solution can be drawn up in threads which become brittle on cooling, the pieces are taken out of the cup, and laid for some days on the ware to harden quicker. When they have become so hard that the edge of the finger-nails makes no impression, cut the pieces into thin sections with a saw, and grind them smooth and flat on an ordinary sharpening stone. Then cement the plates by their smooth sides to a slide by means of Canada balsam or copal varnish, and lay them again on the warm plate. After some days, when the preparation has become firmly fixed, grind it first on a revolving grindstone (or a flat one), and then on a sharpening stone until the section has acquired the right thinness. Wash the section well with water and add Canada balsam, and cover with a covering glass.

If it is desired to show small quantities of organized substances in calcified tissues, the section is treated as above; but before the covering glass is put on it, it should be placed in chloroform till all the resin is drawn out, then carefully decalcified, and last of all coloured. The organic parts can be represented still more beautifully and without the least change of position if the section, as described above, is freed of resin, then cemented with very thick Canada balsam to a slide, and the exposed half only carefully decalcified, then washed and stained. By this means he succeeded in showing, for example, the most delicate connective-substance-lamellæ in the skeleton of Isis elongata.**

Measurement of the Dihedral Angles of Microscopic Crystals.M. Em. Bertrand, whose paper on this subject, presented to the French * Zoologischer Anzeiger,' vol. i. p. 36.

Academy, will be found at p. 217, has contributed some further remarks on the subject to the 'Bulletin of the Mineralogical Society of France,' of which the following is a translation, omitting those parts which repeat what has already appeared in the Comptes Rendus.'

"The eye-piece which enables the observer to make sure that one of the faces of the crystal has its projection perpendicular to the zero plane (described in the previous article), has been slightly modified with the view of obtaining greater sensibility. It is composed of a cylinder of flint of 6 centimetres long, to the middle of which is fixed, by Canada balsam, a plate of crown of the th of a millimetre thick. The flint having a greater, and the crown a smaller refractive index than that of the balsam, it will be seen that the upper part of the cylinder being placed at the focus of the upper lens of the eye-piece, two reticles will appear very close and parallel, and the interior of these two reticles will be illuminated if the face of the crystal has its projection perpendicular to the zero plane of the microscope. However little the crystal is turned to the right or left from this position, the part comprised between the two reticles will cease to be illuminated, whilst the exterior part will be more strongly illuminated either on the right or left according as the crystal has been turned.

The possible error is given by the value of the angle whose sine is al; this angle is less than 10', and as two readings can be made by turning the crystal to the right or left successively until the interval between the two reticles is completely darkened, the error is reduced to 5'.

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Measurements made on crystals of 2 to 3 of a millimetre have given results correct within 6'.

The smaller the face of the crystal is, the greater, of course, will be the sensibility of the process. If a face is observed which is not very small, this face will reflect light into the microscope obliquely to the optic axis of the apparatus even when the projection of the face of the crystal on the horizontal plane is perpendicular to the zero plane, and this oblique light will destroy the clearness of the phenomenon; whilst if the face is very small, all the light reflected by it will be sensibly parallel to the zero plane of the microscope. Moreover, when the face of the crystal is not very large, its image is seen in the microscope on both sides of the double reticle when this face has its projection perpendicular to the zero plane; but on a slight rotation of the stage to one side or the other of the correct position, the image disappears either to the right or the left of the double reticle, and remains visible on one side only. This disappearance of one half of the image in conjunction with the extinction of the part comprised between the two reticles renders the observation very easy. It is sufficient therefore to employ magnifying powers proportionate to the size of the crystal, so that the sides of the face observed should appear in the microscope to be about 2 mm. difficulty in the use of high powers is their short focus, which prevents good illumination. Crystals of about 1 mm. may be measured, but for smaller ones the process ceases to be applicable."

The

M. Bertrand also explains his mode of illumination to be to place a luminous slit of about 30 centimetres long before the microscope

very exactly in the zero plane, which illuminates the crystal from the horizontal direction up to one of about 70°. By a mirror applied horizontally to the cube the crystal can be 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, b, c, and two of the angles a, ß, 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 Refraction.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 24 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 obscurities 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 ordinary 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

the muscles of flight; this base vibrates and the buzzing is produced. But all buzzing ceases if any movement of these organs is rendered impossible, by holding the wings tightly pressed together over as large a surface as possible, so as to exert a certain strain on their base. In whatever manner the wings are held, provided that they are completely motionless, the buzzing ceases absolutely, contrary to the opinion of Hunter.

2. By removing the scaly parts with which the circumference of the stigmata is fringed, the buzzing, far from being annulled, as Chabrier affirms, is not in any way modified, provided that the operation has not weakened the animal sensibly.

3. The respiratory organs may be injured in different ways, and more or less seriously; solid bodies of considerable size may be introduced, without preventing the buzzing or changing its timbre.'

4. If the thoracic stigmata are hermetically closed, as Burmeister has done, the buzzing is not in any way extinguished; it is only weakened in proportion to the weakening of the flight itself.

There are then produced, especially among the Diptera, effects which deserve notice. The animal becomes slow and lazy; it no longer willingly flies. If it does so, its ill-sustained flight is not long before it stops, then the insect succumbs and no longer gives signs of life.

I once saw an Eristal (E. tenax), which having briskly escaped from my fingers, towards the window, immediately after the closing of the stigmata, fell motionless at my feet, entirely exhausted by a flight of a few centimetres. This result does not always follow so suddenly, but it never fails to supervene after a few repeated attempts at flight. It is easily explained by the complete absorption of the provision of oxygen contained in the trachea of the thorax, in consequence of the contractions of the muscles of flight. It is a true asphyxia. At the expiration of some minutes, however, the fly returns to life, owing to the influx of air through the abdomen into the thorax. The animal may then again attempt to fly, or at least to walk, but it is never long before death finally supervenes. These effects are so constant and so easily obtained, that it is really surprising that no experimenter has noticed them.

The causes of the buzzing certainly reside in the wings. It has been recognized for a long time that the cutting of these organs, effected more or less close to their insertion, exercises a more or less marked influence on the buzzing. It becomes thinner and sharper; the timbre itself is considerably modified. It loses the mellowness due to the friction of the air on the edges of the wings, and becomes in some degree nasal. The timbre perceived under these circumstances recalls that of certain reed instruments, or, better still, that of certain electrical contact-breakers, and in no way resembles the sound which the passage of air through an orifice may produce. This sound, on the contrary, agrees entirely with the repeated beatings of the wing-stump against the solid parts which surround it, or of the horny pieces which it contains (osselets radicaux of Chabrier) against each other.

If a slightly fluid substance, which only dries slowly in the air, is coated over the wing-stump of an animal, operated upon in the manner just described, the preceding sound is sensibly deadened, without the stigmata being in any way modified, or the movement of the wings impeded.

When the section includes the stump itself, the sound becomes sharper and weaker. It ceases as soon as a sensitive part is reached; but, as may easily be made evident, it is only because the animal then ceases to execute movements which have become painful.

To sum up, in the Hymenoptera and the Diptera the buzzing is due to two distinct causes; the one being the vibrations of which the articulation of the wing is the seat, and which constitute the real buzzing; the other, the friction of the wings against the air, an effect which more or less modifies the former. It would not be impossible, after these data, to reproduce artificially the buzzing of these animals, and I have some hope of succeeding.

Among the powerful-winged Lepidoptera, such as the Sphinxes, the soft and mellow humming which these animals make is only due to the rustling of the air by their wings. This sound, always grave, is the only one produced; it is not accompanied by basilary beatings, owing to a special organization, and chiefly to the presence of scales.

Among the Libellulæ also, the base of whose wings is furnished with soft and fleshy parts, there does not exist any real buzzing, but a simple noise due to the rustling of the organs of flight.” *

Although the following paragraph has now gone the round of the provincial (and some London) papers, having appeared in the Times' of 17th September, it will not be inappropriate to reprint it here in juxtaposition with the above note of M. Perez which appeared in the Comptes Rendus' of 2nd September:

"The old naturalists thought generally that the buzzing of insects was produced by the vibrations of the wing, but they had scarcely attempted to analyze this phenomenon, and their opinion was abandoned when Réaumur showed that when the wings are cut a blow-fly continues to buzz. Other explanations of the phenomenon have been advanced by various naturalists, but none of them are satisfactory. M. Jousset de Bellesme has been making some investigations on the subject, and, after proving that previous theories are unsatisfactory, he describes the results of his own researches. To avoid confusion, it should be distinctly understood what is meant by buzzing. In the scientific acceptation it means to imitate the sound of the humble-bee, which is the type of buzzing insects. But the humble-bee gives out two very different sounds, which are an octave of each other-a grave sound when it flies and a sharp sound when it alights. We say, then, that buzzing is the faculty of insects to produced two sounds at an octave. This definition limits the phenomenon to the Hymenoptera and the Diptera. The Coleoptera often produce in flying a grave and dull sound, but they are powerless to emit the sharp sound, and consequently do not buzz. There are two or three ascertained facts which will serve as guides in the interpretation of the phenomenon. First, it is indis'Comptes Rendus,' vol. lxxxvii. p. 378.

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