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The daylight which falls upon the mirror R passes through the double prism A B, closed for the purpose of measurement, into the telescopeF: the arrows indicate the direction of the circulation of hot water round the prisms: in order to save space, the thermometer is only shown in part. The magnifier L is fitted with a plaster reflector, the latter not shown in the figure. Bibliogtraphy:
The above three papers deal with the theory and the construction of the refractometers. The voluminous literary matter on the various methods of refractometrical investigation has been collectred in the form of a special refractometrical bibliography, copies of which we shall be glad to supply gratis on application. |
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Object and Scope. Method. Construction and Manner of Action. |
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The double prism (A B, Fig. 1) is opened out and closed up by means of a screw head v, which acts in the manner of a bayonet catch. In order to apply a small quantity of fluid to the prisms, without opening the casing, the screw v is slackened and a few drops of fluid poured into the funnel-shaped mouth of a narrow passage (not seen in the figure). On again tightening the screw, the fluid is distributed by capillary action over the entire space between the two prisms. This arrangement facilitates the investigation of even rapidly evaporating fluids, such as the etheric solution of milk fat employed in the refractometrical determination of the proportion of fat in milk. - In the case of viscous fluids (resins, &c.) a drop of moderate size is applied with a glass rod to the dull prism surface, the double prism being opened out for the purpose. The prisms are then closed up again and before the measurement is proceeded with, the refractometer is left standing for a few minutes in order to compensate any cooling or heating of the prisms which occurred while they were separated. The fitting for heating the prisms of the refractometer is constructed in its essential parts on Dr. R. WOLLNY'S plan of enclosing the prisms in a metal casing with double walls through which water of a given temperature is circulated. A similar heating appliance is fitted to our butter and our milk-fat refractometers. The thermometer provided registers the temperature of the water circulating through the prism casing at its point of exit. The attainment of a constant temperature in the current of water is facilitated by the use of a Spiral Heater and a Water pressure Regulator. The spiral heater is shown in Fig. 2. The water impelled by the direct or indirect pressure of the source of supply passes at a uniform rate of speed through a heated long copper pipe. The spiral heater, about 3 1/2 metres in length, is housed in the space between two telescoping metal cylinders. The inner cylinder is provided with a copper bottom, through which the heated air generated by a Bunsen burner, or a petroleum or spirit lamp, is evenly distributed and conducted to the copper pipe. The upper end of the apparatus bears a coarse wire grating by which the heated air escapes. The objects to be examined (fats, &c.) may be placed upon this grating for the purpose of melting or preliminary warming. The top of the spiral heater is connected with the tube joint D of the refractometer by a short length of tightly stretched tubing, which should not run |
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horizontally, but should rather incline upwards in the direction of D. In this way the accumulation of air bubbles, which would obstruct the uniform flow of water, is checked. In order to be able to watch the presence of air bubbles, as well as observe the speed of the current, it is a good plan to fit a piece of glass tubing between E and the cistern B. This, too, should tend in an upward direction and be connected at E and B by means of very short joints of flexible tubing. If the fall from cistern B to the sink is slight, the use of a glass or metal pipe again commends itself. As already mentioned, slack or kinked tubes increase the already great resistance encountered by the current of water in its long passage, and should therefore be absolutely avoided. To facilitate an immediate suspension of work, it is advisable to fit a stopcock at an easily accessible point (a screw stop .without spring action, by preference). On screwing up the stopcock and turning off both water and gas, everything will come to a complete stand within a minute; similarly, the same temperature will be restored within from 5 to 10 minutes on resuming work, provided the gas tap is regulated exactly as it was before, to facilitate which the gas tap should be provided with a long key. . It is of advantage generally not to have too sluggish a flow of water, to first obtain, a certain approximate temperature by appropriate manipulation of the lamp burner, and finally to regulate the heat to the cxact degree required by varying the difference in elevation between cisterns A and B of the water-pressure regulator. The required degree of temperature having thus been obtained, the tap of the supply pipe is turned off so as just to allow a thin, barely continuous, stream of water to issue from the overflow pipe of cistern A. The difference in elevation between the two cisterns A and B may be varied in two ways; cistern A is either suspended from a cord running on a roller, the free end of which is made fast as with roller blinds, or recourse is had to the contrivance shown in Fig. 2. Cistern A is hung by two hooks to a board, the latter being made to slide up and down in a frame formed of a board about 1 metre in length, fitted with two strips along its sides, the whole suspended against the wall. Holes are bored through this board, arranged in zig-zag lines and each about 1 cm higher than the next lower one, the board bearing cistern A being kept at the desired elevation by inserting a peg into one of the holes. It is an easy matter to find experimentally the number of holes by which the board requires moving in order to cause the temperature to vary by 1°. As the cost of packing and carriage of such a frame would be quite out of proportion to its commercial value, we do not undertake to supply it, but any, carpenter would readily produce one at short notice. If the requirement of a very high temperature (60° C, or more) should necessitate the employment of two spiral heaters, these should not be arranged one behind the other, but side by side. |
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It is comparatively easy to keep the temperature of the water circulation constant to within I or 2 tenths of a degree for a prolonged period. When using gas, a gas-pressure regulator is not an absolute necessity, as variations of pressure, such as may be caused by the opening of a neighbouring gas tap, make no appreciable impression on the temperature of the circulating water. More pronounced varitions, however (for instance, those occurring towards nightfall, or at the beginning and suspension of work in a large factory), make themselves felt in a more disturbing manner, and may necessitate the use of a gaspressure regulator.
Latterly the ABBE refractometer with heatable prisms has been extensively applied also to the purposes for which WOLLNY's butter-refractometer and our milk-fat refractometer are primarily designed. To assist in this case the owner of an ABBE refractometer in the conversion of the nD values ascertained into equivalents of the scales of the butter or milk-fat refractometers, or vice versa, we have compiled a set of conversion tables, which are supplied with each specimen of the ABBE refractometer.
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For the examination of solid and plastic bodies one of the prisms is removed by pressure on a nickelled spring (f, Fig. 6), as it is only required for the measurement of fluids. The measurement is carried out by reflected or grazing incident light.
Index of solid and plastic objects by reflected light. |
Solid bodies are ground and polished so as to produce a
good plane surface. By means of the alidade and the sector the
telescope of the refractometer is inclined to a sufficient degree
to bring the rectangular polished face of the prism into a horizontal
position.
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By removal of the flap the ground surface of the prism has become exposed. The light striking this surface is thence directed into the telescope by reflection on the polished surface, thus also on the object. The appearance of the border line now becomes less clearly defined than in the case of a stratum of fluid between two prisms, because the darker portion of the field of view is illuminated by the partially, the brighter portion by the totally, reflected rays. The most favourable condition of illumination must be ascertained empirically by revolving the entire sector. Carful achromatisation of the border line is most essential in this method. In other respects the rules given on page 4, regarding the reading on the sector, and on the drum of the compensator, are applicable. With small objects or badly polished surfaces a high degree of accuracy demands a series of independent measurements.
After each measurement the surface of the prism must be most carefully cleaned xvith soft linen saturated with a suitable solvent (alcohol, ether, benzol), according to the nature of the substance examined. Want of cleanliness induces an indistinct appearance of the border line, and may even result in false readings of the refractoinoter.
(for solid objects only). |
In the case of viscous
plastic bodies (resin, &c,) a piece about the size of
a pea is applied to the prism and a glance through the square
end of the prism will show whether the substance adheres closely
to the surface without air blisters intervening. A slight preparatory
warming of the substance may sometimes be of advantage. The great
adhesion of such substances renders the use of the flap K
superfluous.
mirror enters the object parallel to the polished
surface of the prism. The appearance of the border line is the
same as in the measurement of fluids by transmitted light (p.
4), namely, considerably clearer than in the method described
under A.
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As shown in Fig. 7, the double prism is opened out and the object joined to the polished surface of the prism by means of a small quantity of monobromonaphthalene. The movable prism is hinged down, so far that the light from a clear sky which strikes the bright metal surface F enters the object in the manner indicated. Care should be taken not to mistake the image of the edge of the beam of a window frame or of the ridge of a roof for the border line. In cases of doubt a piece of white paper spread over the surface F will slightly obscure the field of view, when the border line will be recognized without fail.
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After
having previously removed the lower prism (see p. 9), a few drops
of the fluid are applied to its ground surface. It is then reinstated,
sliding the ground surface along the other one until the nickelled
spring f is clearly heard to snap into its notch. Having ascertained
by a glance into the square polished surface of the prism that
the fluid fills up the entire space between the two prisms, the
measurement proceeds in the usual manner. The double prism is
manipulated as described on page 9 and the specimen applied to
the horizontal polished prism surface only on occasions when
extremely liquid or volatile substances require to be dealt with.
We supply with each
refractometer a testing plate made of glass of known refractive
index, the value of which is marked on the upper ground surface
of the plate. The plate has two plane polished surfaces which
form a sharp edge. For the purpose of testing the refractometer
the refractive index of the plate P is very accurately measured
by applying it several times in succession, the prism surface
being wiped clean after each measurement. The mean of the several
measurements is compared with the value stated on the plate,
and if the 4th decimal, the adjustment of the refractometer may
be considered satis-
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factory. More pronounced discrepancies suggest the probability of some component part of the refractometer having become deranged, and in such a case it is always advisable to forward the instrument to our works for readjustment. Owners are cautioned against any attempt on their own part at correcting the instrument or entrusting it to any mechanician, however skilful, for repairs instead of applying to us. Experience has taught that, with careful treatment, derangement of the refractonieter is improbable, even after years of wear.
As the border line cannot be rendered colourless it is necessary to employ monochromatic light (LOWE's sodium burner, or SIEDENTOPF'S mercury arc lamp). This refractometcr is peculiarly suitable for demonstration from the reason that the optical constants of the apparatus required for the measurement of refractive indices - the angle of refraction and the refractive index N of the flint prism - have to be determined on the apparatus itself, and because the three methods described under A, B, C on pages 9 - 11 and the differences in the appearance of the border line peculiar to them - more particularly the quality of the colour band under white light, and the distribution of light on both side of the border line - are immediately demonstrable. The refractometer facilitates the determination of the refractive index of solid and fluid bodies, ranging from refractive index 1 up to nearly 1.7, that being the index of the flint prism. * See C. PULFRICH in "Zeitschr. f. Instrumentenkunde". Vol. 18, 1898, pp. 114 et seq. |
This refractometer
embodies only the parts required to explain the nature of the
ABBE refractometer, viz: the ABBE double prism, the telescope
with reticle, and a sector graduated in 1/2
° and provided with a nonius, while the compensator and the
heatable prism casing are omitted.
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 Fig. 10-12 indicate the path of the rays in the observation of total reflection on air used for the measurement of the refractive index of the flint prisms.
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representing an
auxiliary angle. The value of N will then at once be found
to be
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