70. The Laurent Burner.—The theory of the illumination of the laurent burner is illustrated by the accompanying [Fig. 33]. The lamp consists essentially of two bunsens, surmounted by a chimney.[38] A curved spoon made of platinum gauze serves to hold the fused particles of sodium chlorid which are used to produce the yellow light. The spoon is shown at G, held by the arm F, fastened by the key P. The interior intense flame B B is surrounded by an exterior less highly colored flame A A. It is important that the optical axis of the polariscope be directed accurately upon the disk B, which is the most intense part of the illumination. The point of the spoon carrying the salt should be coincident with the prolongation of the lamp TT, so that it just strikes the edge of the blue flame. Care should be taken not to press the spoons into the interior of the flame as by so doing the intensity of the illumination is very much diminished. Great care must be observed in the position of the spoon G, and the platinum arm F being flexible, the operator with a little patience, will be enabled to properly place the spoon by bending it. Moreover, if the spoon be pressed too far into the flame, the melted particles of salt collecting in the bottom of G may drop into the lamp and occlude the orifices through which the gas enters. The light of the yellow flame produced by the lamp may be further purified by passing through a prism filled with a solution of potassium dichromate, or better, a homogeneous disk cut from a crystal of that salt.
Since the flame produced by the above device is not perfectly constant, being more intense at the moment of introducing a fresh portion of the fused salt, the author has used a lamp designed to furnish an absolutely constant flame.[39] This device which is shown in [Fig. 34], is based on the principle of adding constantly a fresh portion of the salt to the flame. The flame is thus kept perfectly uniform in its intensity.
The lamp consists essentially of two wheels with platinum gauze perimeters and platinum wire spokes, driven by a clock-work D, and mounted by the supports AAʹ as shown in the [figure]. The sodium salt, chlorid or bromid, in dilute solution, is placed in the porcelain crucibles F, supported by BBʹ as indicated in the figure, to such a depth that the rims of the platinum wheels dip beneath the surface as they revolve. The salt is volatilized by the lamp E. By means of the crossed bands the wheels are made to revolve in opposite directions as indicated by the arrows. The solution of the salt which is taken up by the platinum net-work of the rim of the wheel, thus has time to become perfectly dry before it enters the flame and the sputtering which a moist salt would produce is avoided. At every instant, by this arrangement, a minute fresh portion of salt is introduced into the flame with the result of making a perfectly uniform light which can be used for hours without any perceptible variation. The mechanism of the apparatus is so simple that no further description is necessary. The polariscope should be so directed toward the flame as to bring into the field of vision its most luminous part. The platinum wheels are adjustable and should be so arranged as to produce between them an unbroken yellow flame. The wheels are eight centimeters in diameter and are driven at a rate to make one revolution in from six to ten minutes.
Figure 34. Lamp for Producing Constant
Monochromatic Flame.
71. Construction of Laurent’s Apparatus.—The shadow polariscope invented by Laurent is constructed as follows: The polarizer is a special nicol which is not fixed in its position, but is so arranged as to be turned through a small arc about its axis. By this device, the quantity of light passing through it can be regulated, and the apparatus is thus useful with colored solutions which are not easily cleared by any of the common bleaching agents. The greater the quantity of light admitted, however, the less delicate is the reading of the shadow produced. The plane of polarized light emergent from this prism, falls on a disk of glass half covered by a thin lamina of quartz which thus divides the field of vision into halves. It is this semi-disk of quartz which is the distinguishing feature of the apparatus.[40] The polarized light thus passes without hindrance the half field of vision which is covered by the glass only, but can not pass the quartz plate unless its axis is set in a certain way. The field of vision may be thus half dark, or both halves may be equally illuminated or equally dark according to the position of the nicol analyzer which is freely movable about its axis and carries a vernier and reading glass over a graduated circle. The field of vision in the laurent may have any of the following forms.[41] Let the polarizer be first so adjusted that the plane of polarization of the transmitted pencil of light is parallel to the axis of the plate lying in the direction A B. The two halves of the field of vision will then appear equally illuminated in every position of the analyzer. But if the polarizing nicol be inclined to AB at an angle a, the plane of polarization of the rays passing through the quartz plate will undergo deviation through an equal angle in the opposite direction.
Figure 35. Field of Vision of a Laurent Polariscope.
It happens from this, that when in the uncovered half of the field, the plane of polarization has the direction AC, in the other half it will have the direction ACʹ. When the analyzer is rotated, if its plane of polarization lie in the direction cc, the rays polarized parallel to AC will be completely extinguished and the corresponding half of the field will be dark. The opposite happens when the plane of polarization lies in the direction of cʹcʹ. When one-half of the field is thus obscured, the other suffers only a partial diminution in the intensity of its illumination. When the middle position bb is reached in the rotation of the analyzer, the illumination of the two halves is uniform, and this is the point at which the zero of the scale is reached. The slightest rotation of the analyzer to the right or left of this neutral point will cause a shadow to appear on one of the halves of the field, which by an oscillatory movement of the analyzer, seems to leap from side to side. The smaller the angle a or BAC, the more delicate will be the shading and the more accurate the observation. This angle being adjustable by the mechanism already described, should be made as small as will permit the admission of the quantity of light requisite for accurate observation.
The various pieces composing the polariscope are arranged in the following positions, beginning on the right of [Fig. 36], and passing to the left, where the observer is seated.[42]