First-order red plate

The use of a single polar is compatible with phase contrast microscopy. Crossed polars produce a dark field in which fine fibers will not be seen. If a compensator such as a first order red plate is also used, most of the fine fibers will be seen provided the light source (31) is sufficiently intense. 

The optical behavior of asbestos fibers viewed with crossed polars has been described. Crystalline fibers have positions of extinction 90° apart. The fact that crystalline fibers have retardation has also been mentioned. With crossed polars and a first order red plate in place, asbestos fibers will go from yellow to extinction to blue, back to yellow, etc., upon rotation of the stage. If the fiber bends, this is equivalent to a rotation of the stage and the color will change. If the fiber... 

Plant fibers will show cells of color with the first order red compensator. The complicated color pattern reveals the complicated internal structure of the plant fiber and shows it to be a non mineral fiber. Occasionally, plant fibers show a single color when examined with crossed polars and first order red plate. In such cases, close observation of morphology or dispersion staining can be used to make a distinction. 

It appears likely that we observed band structures during shear due to an inadvertant superposition of oscillatory and steady shear. An attempt should be made to reproduce a quasi-steady state band structure and to explore this phenomenon. Either this will turn out to be relatively easy to do, or we were unbelievably lucky (yet again ) to observe it. If this is accomplished, it would be interesting to insert a first order red plate at various angles to the shear direction. 

Phase-Polar Microscope The phase-polar microscope is a phase contrast microscope which has an analyzer, a polarizer, a first order red plate and a rotating phase condenser all in place so that the polarized light image is enhanced by phase contrast. 

In white light, anisotropic structures may appear brightly colored when viewed in crossed (or parallel) polars. These polarization or interference colors depend on the retardation (Section 3.3). An estimate of sample retardation can be made from the standard sequence of colors, published as the Michel-Levy chart in many texts [4, 7, 9, 17, 18]. Color can also be used to find the sign of birefringence when a first order red plate is inserted as a compensator in white light. 

The first order red plate is also known as the sensitive tint plate. Its main use is in determining the sign of the birefringence of objects of small retardation, e, which have a grey color in crossed polars. With the first order plate in place the net retardation is (575 + e) nm, which is second order blue, or (575 - e) nm, which is first order yellow. The eye is very sensitive to color changes in this range, so a very small value of e has a visible effect. 

Fig. 3.14 Two images of a thin i melt-cast film of polycaprolactone. As in Fig. 3.13, the region is 200 x 250/zm, the left-hand image was taken under crossed polars and for the right-hand image, a first order red plate was also used. The spherulites in this material are much less regular, and some show colors under crossed polars. The first order red plate image can still show that the spherulites are negatively birefringent.

Figure 3.15. Two images of a thin melt cast film of high density polyethylene the region is 200 x 250//m. The left hand image was taken in crossed polars. The radial Maltese cross is due to the extinction position. The spherulites in this material have dark circumferential bands. The crystals twist as they grow, and their orientation in these bands has the optic axis perpendicular to the specimen plane. The right hand image is the same area when a first order red plate is also used. The blue and yellow colors show that the spherulites are negative. (See color Insert.)...

Figures 3.15 and 3.16 show spherulites in crossed polars and with the sensitive tint plate. In crossed polars, the spherulites are bright except at the four perpendicular radial directions where the crystals are in the extinction position. High density polyethylene (see Fig. 3.15) produces spherulites that are unusually perfect, and also have circumferential dark bands. The bands are regions of apparent isotropy where the optic axis is perpendicular to the specimen plane. With the first order red plate placed so that its slow direction is at -l45° (top right to bottom left of the image), the...

Various compensators such as a qrrartz wedge, a first-order red plate, or a quarterwave plate can be introduced into the bodytube. Compensators aid in the detemrination of degree of retardation or birefiingence and in detemrin-ing the orientation of the high refractive irrdex. 

Figure 2 Spherulites in a thin film of high-density polyethylene in crossed polars with the first-order red plate. The pattern of blue and yellow quadrants shows that the spherulites are negative. The circumferential dark rings are the loci of low birefringence, where the twisting lamellar crystals have the molecular chain axis parallel to the axis of the microscope. Scale bar=20pm.

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