Try each of the following methods of mounting objects, and if successful, cover the slides with paper and label them, giving also your name and the date. Unless the object is very thin, or if it is liable to be injured by pressure, it must be protected by a cell. This consists of a circular or square enclosure, on which the thin glass plate is laid, so as to leave a space between it and the slide. Cells may be made of various materials, as paper, cardboard, or tinfoil, and fastened to the glass by gum. These are very convenient for mounting objects dry, especially such as are not injured by the air. Mount in this way some crystals of bichromate of potash. Shallow cells may be made of Brunswick black, applying it with a brush. They are best made in a circular form by Shadbolt's apparatus, in which the slide is placed on a small turntable, which is made to revolve rapidly by drawing the forefinger of the left hand over a milled head attached below, while the brush is held in the right hand. If the plate is warmed, the black will dry rapidly, and the thickness of the cell may be increased by applying successive layers. Make several such cells for some of the objects to be mounted in balsam, as described below. To preserve a liquid, or an object of considerable size, thick cells are employed, which may be procured ready-made of glass. They may be cemented to the slide by marine glue, warming them sufficiently to melt it, removing the superfluous glue by a sharp knife, and washing it clean with a solution of potash. Fill such a cell with some liquid, as vinegar, and fasten on the cover with marine glue. Take great care thatno air bubbles enter, and that the joints are perfectly tight.

The best method of mounting the parts of insects, sections of wood or bone, and in fact most substances, is in Canada balsam. The object, as the foot of a fly, must first be dried and freed from air-bubbles. For this purpose it should be heated nearly to the boiling point of water, or placed under a bell-glass containing concentrated sulphuric acid. To remove the air it should be soaked in turpentine and gently warmed; a much more effective method is to place the whole under the receiver of an air-pump and exhaust. Now lay the slip of glass on a little stand of brass, and heat it by means of a spirit-lamp, or Bunsen burner. Take up a little Canada balsam on the end of an iron wire, and lay it on

the slide, when the heat will render it perfectly fluid. Pick up the object on the point of a needle, immerse it in the balsam, and then cover it with a piece of thin glass. Great care must be taken that both slide and covering-glass are perfectly clean, and that no dust gets into the balsam, as otherwise the object will be much disfigured when viewed under the microscope. The main difficulty is to prevent air-bubbles remaining on the slide. If present, they may be removed by a cold wire, or burst by touching them with a hot needle. The covering-glass must be lowered into place very slowly, or bubbles will adhere to its surface. The whole is then put away to harden under pressure, and the superfluous balsam afterwards removed by the aid of a little turpentine.

The structure of objects of large size is generally best seen by cutting thin sections of them, so that they may be rendered nearly transparent, and be viewed by transmitted light. Soft substances, as vegetable or animal tissues, may be cut with a sharp knife or scissors, or better, with a Valentin's knife, which has two parallel blades whose distance apart may be varied by a screw. They should be well wet with water or glycerine, or the section will adhere to them.

Harder substances, as wood or horn, are cut in thin sections by forcing them through a hole in a thick brass plate, cutting off the projecting portion, pushing it through a little farther, and cutting again. By means of a screw, sections of any desired thickness may thus be obtained. Cut longitudinal and transverse sections of a piece of pine wood, first soaking it in water, and mount them in Canada balsam. Cut also some thin transverse sections of hair by fastening a number of them together with gum so as to form a solid mass; cut a thin section, and then dissolve the gum in water.

To cut a thin section of still harder substances, as bone, quite a different method must be employed. A thin piece is first cut off with a fine saw, such as is used for cutting metals; it is then filed thinner, and finally ground down to the required thickness with water between two hones. On examining the section thus obtained, it will be found covered with scratches, which must be removed by grinding it on a dry hone, and afterwards polishing it on a sheet of plate glass. Prepare two such sections, soak one in turpentine

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and remove the air by means of a pump, and then mount both in Canada balsam. The difference in their appearance will be very marked, the one from which the air has not been removed appearing full of black spots or lacunæ, formerly called bone corpuscles. They are really cavities filled with air, which in the second specimen is replaced by the turpentine.

This experiment is well supplemented by performing some dissections of animal and vegetable substances, injecting tissues, and mounting thin sections of them.

82. FOCI AND APERTURE OF OBJECTIVES.

Apparatus. Two instruments are needed for this Experiment. First, a microscope with a positive eye-piece, a spider-line or eyepiece-micrometer, and a stage-micrometer, also several objectives to be measured. To measure the angular aperture, a graduated circle is employed with an arm and index, to which is attached a short brass tube, like the body of a microscope. The objective to be tested is screwed into one end of this tube, and a positive eyepiece slipped into the other. The tube is made so short that when the objective is directed towards a distant object, the image formed may be viewed by the eye-piece. To obtain a higher magnifying power, the eye-piece may be replaced by a compound microscope, like that used in Experiment No. 20. To obtain an accurate measurement when the object observed is not very distant, it is essential that the end of the objective should lie in the axis of the circle. This is most readily accomplished by means of a ledge, on which a vertical plate of glass may be placed with its front face over the axis of the circle. The objective is then brought up in contact with it, the tube clamped, and the glass removed.

Experiment. To measure the focal length of an objective it is assumed that two of the laws of simple lenses hold for a compound lens. First, that the sum of the reciprocals of the conjugate foci equals the reciprocal of the principal focus, and secondly, that the ratio of the magnitudes of the object and image equals the ratio of the conjugate foci. This assumption is not strictly correct, and valuable work might be done in determining the amount of the deviation. Screw the objective to be measured upon the microscope, and measure the divisions of the stage-micrometer, with the spider-line micrometer. Reduce to absolute measure

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FOCI AND APERTURE OF OBJECTIVES.

ments from the magnitude of the parts of the micrometers, or if these are not given, determine them from the Dividing Engine, Experiment No. 21. This reduction may be avoided by using two similar eye-piece micrometers, A and B. Measure several divisions of A with B, and call the mean of the readings m. Measure, in the same way, B with A, and call the mean reading m'. The true reading, n, will be the mean proportional of these two. Of course if the micrometers are precisely alike, m will equal m'. Now measure the distance between them, and call the distance D. Then iff equals the focal length of the objective, and f', f" its

which ƒ D and knowing D and n, f may be deduced. (n + 1)2, The number given by the maker is generally greater than the true focal length of the objective, and this experiment affords an excellent means of correcting it. To show the value of such measurements, and the accuracy attainable by them, see an article by Prof. Cross, Journ. Frank. Inst., Vol. LIX., p. 401. Useful work might also be done by varying D, and noting the effect on f, also by changing the correction for cover, or distance between the lenses.

To measure the angular aperture of an objective, screw it into the end of the tube attached to the graduated circle, set a plate of glass on the ledge, and bring the objective against it. The front surface of the lens will then be just over the axis of the circle. Now clamp the tube, remove the plate of glass, and slide the eyepiece or small compound microscope into place. Bringing it near the objective, an image of outside objects is seen, the whole in fact, forming a telescope with the objective for an object-glass. The field of view is seen to be bounded by a circle whose true angular diameter gives the aperture of the objective. Select some strongly marked vertical line, as the sash of the window, and notice that as the objective is turned from side to side, the image of this line moves also. Bring it to coincide first with one edge of the circle, and then with the other. The difference in the reading of the index in the two cases equals the angular aperture. Repeat this measurement with several other objects.

83. TESTING PLANE SURFACES.

Apparatus. A stand carrying two telescopes, which may be placed opposite each other, or set at right angles. The eye-piece of one, which acts as a collimator, is replaced by a plate of brass pierced with a very fine hole. This is placed exactly at the focus of the object-glass, and being illuminated by a lamp, forms a bright point of light or artificial star. The optical circle might be used for this experiment, but the graduated circle is not needed, and it is better to have telescopes of larger size. A millimetre scale is also wanted, a prism, a sextant-glass, a piece of plate or window glass, and a lens of very long focus.

Experiment. Make the same adjustment for parallel rays as is described in Experiment 72. That is, focus the observing telescope carefully on some distant object as a star, and turn it toward the collimator. An image of the hole or artificial star at the further end of the latter will now be visible, but it will generally be out of focus. Draw it towards, or from its object-glass until accurately focussed, when it should appear as a very minute circle of light, like a star. Measure with the millimetre scale the distance between two points, one on the eye-piece, the other on the end of the tube in which it slides. Throw the star out of focus by moving the eye-piece, and focus again; repeat ten times, and take the mean of the distances between the two points. Now set the telescopes at right angles, and place the surface to be tested at the intersection of their axes, equally inclined to each, and vertical. The image of the star reflected in the surface, will then fall in the centre of the field, and if the surface is perfectly plane will be as distinct as that previously obtained, although fainter. In general, however, it will be a little out of focus, due to the curvature of the surface. In this case move the eye-piece, focus ten times as before, and take the mean reading of the distance between the two marked points. Measure the focal length of the observing telescope, or the distance from its object-glass to the cross-hairs, also the angle between the axes of the collimator and observing telescope, unless this is fixed at 90°. Call F the focal length, d the change in position of the eye-piece, or difference of the means of the two sets of ten observations, v the angle between the axes, a the distance from the objective of the telescope to the plane sur