Magnification optical microscopes

The various techniques which may be used to provide optimum conditions for the examination of specimens have been described [202—205]. If the sample is opaque, then microscopic investigation is limited to the surface. The depths of penetration for the study of transparent crystals are controlled by the limited depth of field of the optical microscope at high magnifications. This limitation can sometimes be overcome by cleavage of the crystal at an appropriate value of a and examination of the surfaces exposed [120],... 

An optical microscope photograph taken at 200 X magnification using polarizing filters is shown in Fig. 21. The spherulites show a characteristic Maltese cross pattern produced by the interaction of the polarized light with the... 

The microstmcture of conventional ceramics contains flaws readily visible under optical microscopes the microstmcture of advanced ceramics is far more uniform and typically is examined for defects under electron microscopes capable of magnifications of 50,000 times or more. 

Powerful methods that have been developed more recently, and are currently used to observe surface micro topographs of crystal faces, include scanning tunnel microscopy (STM), atomic force microscopy (AFM), and phase shifting microscopy (PSM). Both STM and AFM use microscopes that (i) are able to detect and measure the differences in levels of nanometer order (ii) can increase two-dimensional magnification, and (iii) will increase the detection of the horizontal limit beyond that achievable with phase contrast or differential interference contrast microscopy. The presence of two-dimensional nuclei on terraced surfaces between steps, which were not observable under optical microscopes, has been successfully detected by these methods [8], [9]. In situ observation of the movement of steps of nanometer order in height is also made possible by these techniques. However, it is possible to observe step movement in situ, and to measure the surface driving force using optical microscopy. The latter measurement is not possible by STM and AFM. 

In a conventional optical microscope, provided the lenses have been ground with spherical symmetry, the magnification is inevitably the same in both horizontal and vertical directions of the image. In a scanning microscope this... 

ELECTRON MICROSCOPE. The concepts that eventually led to the development of electron microscopes came out or the discovery of the wave nature of the electron in 1924. The effective wavelength of the electrons varies with accelerating voltage and is less than 1 A >. = /< 150/ V) A. This short wavelength makes possible far better resolution and higher magnification in the electron microscope as compared with the optical microscope. 

Owing to its large numerical aperture, the depth of focus of an optical microscope is relatively small (c. 10 /xm at x 100 magnification and c. 1 /xm at x 1000 magnification). This is not always a... 

An appropriate selection of the maximum useful magnification of an optical microscope for a given sample is also important. The magnification of the microscope is the product of the objective-eyepiece combination. As a rule of thumb, the maximum useful magnification for the optical microscope is 1,000 times the numerical aperture. Table 1.4 summarizes the maximum useful magnification and the eyepiece required for different objectives. 

In order to visualise the NiBi layer at temperatures below the melting point of bismuth, experiments with Ni-Bi couples must evidently be carried out in the 100-1000 h time range. At such annealing times, its thickness will probably exceed a few micrometres. The NiBi layer formed should therefore be seen even under optical microscope at moderate magnifications. A serious obstacle to performing such experiments may be the rupture of Ni-Bi specimens with thick NiBi3 intermetallic layers. 

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

Metal Particles This test is required only for ophthalmic ointments. The presence of metal particles will irritate the corneal or conjunctival surfaces of the eye. It is performed using 10 ointment tubes. The content from each tube is completely removed onto a clean 60-mm-diameter petridish which possesses a flat bottom. The lid is closed and the product is heated at 85 °C for 2 h. Once the product is melted and distributed uniformly, it is cooled to room temperature. The lid is removed after solidification. The bottom surface is then viewed through an optical microscope at 30x magnification. The viewing surface is illuminated using an external light source positioned at 45 ° on the top. The entire bottom surface of the ointment is examined, and the number of particles 50 pm or above are counted using a calibrated eyepiece micrometer. The USP recommends that the number of such particles in 10 tubes should not exceed 50, with not more than 8 particles in any individual tube. If these limits are not met, the test is repeated with an additional 20 tubes. In this case, the total number of particles in 30 tubes should not exceed 150, and not more than 3 tubes are allowed to contain more than 8 particles [15]. 

The apparatus used for IR microscopy is a Fourier-transform infrared (FTIR) spectrometer coupled on-line with an optical microscope. The microscope serves to observe the sample in white light at significant magnification for the purpose of determining its morphology, as well as to select the area for analysis. The spectrometer, on the other hand, enables study of the sample by transmission or reflection measurement for the purpose of determining the chemical composition. It also provides information about the microstructure and optical properties (orientation) of the sample. It is possible to apply polarised light both in the observation of the sample and in spectrometric measurements. 

Figure 28.22 shows the optical microscopic pictures of the four different scribes at 50 X magnification. Flat scribes were produced by stationary mode (designated as V shape) with horizontal dragging of the cutter tip across the panel surface. 

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