chloride becomes converted into the very dark red ferric acetate. The diatoms are now allowed to settle closely for two or three hours, and the excess of iron salt poured off as completely as possible. Next the test-tube with the moist diatoms is stood in a small vessel of boiling water till all the ferric acetate has passed into the basic state, as evidenced by its changing to an opaque buff colour. The diatoms, now "charged" with the insoluble basic ferric acetate, are shaken up with a few drops of water and acetic acid, and poured into not too great an excess of a solution of potassium ferrocyanide in acetic acid. After standing for some hours with occasional agitation, the excess of Prussian blue which has been formed among and around the diatoms can be removed (at any rate in great part), by repeatedly shaking up with fresh lots of distilled or rain water, as after elimination of the soluble salts this body assumes a form which settles very slowly indeed. Stirring the settled diatoms with a soft camel's-hair brush helps to remove the precipitate which may be clotted on their surface.

(2) Platinum Method.-Applicable to all diatoms, but apt to fail. To the cleaned and ignited diatoms contained in a small porcelain crucible add an alcoholic solution of sodio-platinic chloride, and evaporate with extreme slowness and without any approach to boiling. Finish the drying with the utmost care, to prevent the formation of bubbles of steam within the minute cavities, as this would result in ejection of the platinum salt and the consequent failure of the preparation. When quite dry raise the temperature very slowly till a low red heat is reached, and then throw a few crystals of oxalic acid into the crucible and immediately replace the cover. This completes the reduction of the platinum salt to metal and sodium chloride, and it now only remains to wash away the latter and as much of the unattached platinum as possible, and to select any required diatom from the residue.

(3) Mercurous Sulphide Method. As stated above, this is the best method I have hitherto found for "charging" all diatoms except those having the finest markings. Take a cold saturated solution of mercurous nitrate (sub-nitrate of mercury) and dilute it with its own bulk of water in a small test-tube. Add the diatoms and a drop of metallic mercury, and keep the whole standing corked up for as long a time as can be spared-days are better than hours, and weeks better than days. Shake the tube and withdraw the diatoms suspended in the liquid by help of a pipette, leaving behind any crystals of basic sub-nitrate of mercury which may have formed. Allow the diatoms to settle in a small test-tube, and draw off the supernatant liquid first by a pipette and then by a moistened thread or a very thin strip of filter paper till nothing but a slightly moist mass of diatoms remains. Now add several drops of a strong solution of ammonium sulphide which has been recently prepared, and which is practically free from dissolved sulphur (it should be almost colourless, not yellow), and shake. Fill up the tube with water, cork, and

allow the whole to stand for some hours. Wash and levigate as in the other methods. The mercurous sulphide thus formed is a black amorphous precipitate, which fills the "lacunæ" of the diatoms with an almost completely opaque stopping. Mercuric sulphide is apt to become red and crystalline; hence the necessity of the precautions to avoid the conversion of one into the other, which are detailed above. The only fault of this method is that the sulphide is somewhat apt to clot and become difficult to remove from the outside of the valves by washing. Perhaps this would be avoided by using weaker solutions than those I have worked with.

(4) Silver Nitrate Method.-A strong solution of silver nitrate (about 100 grains to the oz.) may be substituted for the mercurous nitrate, but on the whole does not serve so well except for those diatoms having the finest markings, e. g. Pleurosigma angulatum. The silver sulphide formed is brown and less opaque than the mercurous sulphide, but is not so apt to clot over the surface of the object. By none of these methods will every diatom in a batch be equally well charged.

Diatoms treated by one or other of these methods exhibit very clearly that all "striæ," "dots," &c., are, as stated in the first paragraph, cavities of some kind, which, in default of a better name, might be called "lacunæ "or "pores.'

Whether these lacunæ are complete perforations through the silicious test or mere pits or depressions on the inner or outer surface of the valve, or more or less flask-formed cavities communicating by one or more canals with the inner or outer surface, or with both, cannot at present be resolved with any degree of certainty in the case of those diatoms which have the finer markings. But in the case of some large Coscinodiscs it can be shown that the valve has a structure which may be described as cellular. Where the areola are widely separated from one another, a fragment of a charged valve viewed edgeways presents the appearance of a number of mammæform cells springing from the inner side of the outer face of the valve by their wider extremity, and terminating in a more or less conical perforated apex at the end facing inwards. Fig. 1 shows a valve of this description on the flat. All my edge specimens have spoilt themselves by rolling over.

Where the areola are very close together, so as to cause one another to assume the hexagonal form, the cells which constitute their prolongation partake of the same form, and their inner faces join together to form a perforated plate of considerable substance. The whole structure presents a close resemblance to a single layer of honeycomb cells with their cappings and bases complete but perforated. Fig. 2 exhibits an edge view of a fragment showing this structure.

The outer face or surface of these cells, very commonly if not universally, consists of a thin silicious membrane pierced with a

number of minute holes arranged in a symmetrical manner (constituting the so-called secondary markings), which differs in every species I have observed. Fig. 3 shows a portion of such a capping of one of them.

The cell-walls connecting the two surfaces are exceedingly thin and fragile, and are easily destroyed and lost sight of, while the two plates which they join are comparatively stout, and are often found separate and entire. The details of cell-form vary widely in different species.

In the case of the larger Pinnulariæ, e. g. viridis and nobilis, it can be easily seen that the striæ are pseudo-tubes contained in the walls of the valve, and which may be considered as formed by the lapping towards one another of the edges of a groove sculptured on the inner wall of the valve. I have observed indications of channels of communication between these pseudo-tubes and the outside of the valve, similar to those forming the secondary markings of the Coscinodiscs, but seek further confirmation. Fig. 4 shows a partly charged valve of Pinnularia major (?).

olatun

Of" dotted" diatoms, Cocconema lanceolatum (fig. 5), Stauroneis phonicenteron (fig. 6), and the various Pleurosigma and Naviculæ, all that can be affirmed with certainty is that the dots are hollows. Further experiment is required to determine the point whether they have or have not the same cellular structure as the Coscinodiscs. Mr. Smith has shown that they have two skins or layers; is it not probable that these are connected in the same manner as those of the larger forms? Edge views of fragments of charged Cocconema and Stauroneis seem to show the black sulphide extending as a streak from one face to the other of the single valve, but in the case of such exceedingly minute structure, as is here in question, it is very easy to be misled by one's prepossessions, and it is therefore quite possible that on this point I have been deceived. What precise function these lacunæ or pores fulfil in the economy of the organism, is a question which I hope to study in the immediate future.

VII.-On a Simple Form of Heliostat, and its Application to Photomicrography.

By THOMAS COMBER, F.L.S.

(Read 21st May, 1890.)

YOUR Secretary has asked me to give your Society a detailed description of the apparatus I use for photomicrography, and of my method of working; but it appears to me that it will be simpler and shorter, and at the same time answer every purpose, if I merely explain those features in which my mode of working differs from that which I believe is generally adopted by others, and is probably sufficiently well known. The general nature of the arrangement will be apparent from the woodcuts.

The two main objects that I have endeavoured to attain have been, firstly, a means of sunlight illumination, easily applied, quickly adjusted, and simple in construction so as not to be liable to get out of order; and secondly, an arrangement which admits of convenient and comfortable eye-observation, for the purpose of arranging the object and adjusting cover-correction, before the camera is attached to the Microscope.

So far as my experience goes, for high magnification-other things being equal, both as regards objectives and manipulative skill— better results can be obtained by sunlight than by any other kind of illumination. The photomicrographs produced by Mr. Nelson and other of your members by oxyhydrogen light may be superior to what others have produced by sunlight; but this is due to their superior optical appliances and greater skill as microscopists, which more than compensates for what I cannot help regarding as inferior illumination. The same operator, using the same lenses, will, I am confident, produce better results by sunlight than by any artificial illumination.

The reasons sunlight has been so little used in this country are probably (1) the uncertainty of our climate; (2) the fact that many of our microscopists work chiefly in the evening; and (3) the complicated nature of the heliostats obtainable, which renders them very liable to get out of order, and so difficult to adjust that, when sunlight is available, much time is lost in setting up the apparatus; and, consequently, before everything is in working order, the sun may too often become clouded. The last objection is aggravated by the heliostat being usually placed a considerable distance from the Microscope, and sometimes even outside a window; and, as any error in the action of the heliostat is increased in proportion to the distance, it has been found almost impossible to keep the illuminating beam unchanged by the motion of the sun.

To avoid this difficulty, I place the heliostat inside the window,

and bring it quite close to the Microscope, so that it is within arm's length of the observer, and the sunbeam has so short a distance to pass before it reaches the substage condenser, that any slight error of the heliostat is of comparatively little consequence. The heliostat and all the accessories are fixed, once for all, on a wooden stand, so that they have not to be arranged each time they are used, but the stand has merely to be placed before the Microscope, and everything is in its proper relative position.

The heliostat itself is a brass time-piece A, fig. 47, to which is added an additional motion, causing the spindle, which need not be in the centre, to revolve once in twenty-four hours. It is mounted on a triangular brass plate, furnished with levelling screws, and is fixed at an angle to the horizon, corresponding to the latitude of the place in which it is to be used. When the point of the brass plate is directed due south, and the plate itself is levelled, by means of a spirit-level, in both directions, the clock is in the plane of the equator, and the spindle, at right angles to it, is parallel to the axis of the earth, and points to the North Pole of the heavens. The spindle is made slightly conical, and fitted to it, friction-tight, so as to be capable of easy rotation by the hand, is a small mirror B, with universal motion. The size of mine is two inches by one, which is ample. This mirror has to be set to reflect the light from the sun in the direction of the spindle, when the rotation of the spindle, corresponding exactly with that of the earth, only in the reverse direction, compensates for the apparent motion of the sun, and the reflected beam remains motionless. Where the reflected beam crosses the optic axis of the Microscope, there is placed a second fixed mirror C, inclined to the horizon at an angle equal to half the latitude, which reflects the beam in the axis of the Microscope. Between this fixed mirror and the condenser is placed an alum-cell D, to absorb the heat. In originally fixing the position of the mirrors, care has to be taken that the centre of the fixed mirror is truly axial with respect to the substage condenser and Microscope, and that, reflected in it when viewed through the Microscope, the spindle of the heliostat appears exactly end on, in the centre of the field. The heliostat will then be in its correct position, and the movable mirror can be placed upon it. All this may seem very complicated in the description; but once the position of the various pieces has been thus settled, all that has to be adjusted is the movable mirror, and its adjustment is no more difficult than that of the mirror which forms the ordinary adjunct of the Microscope. If the mirrors are of glass silvered at the back, the first gives a double reflection, which is again doubled by the second, and great loss of light is experienced. Glass silvered on the surface avoids this, but I found it tarnished quickly; so that I have had to adopt reflectors of speculum metal. These also are open to objection, for the light they reflect is distinctly reddish in tinge, and I believe there is considerable absorption of the rays of highest refrangibility.