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Using a phoropter to determine a prescription for eyeglasses. An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.
When visual acuity is below the largest optotype on the chart, the reading distance is reduced until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, they are tested as follows:
Amateur use of VHF and UHF allocations exploded in the late 1960s and early 1970s as repeaters started going on the air. Repeater use sparked a huge interest in the 2-meter and 70-centimeter (420–450 MHz) bands; however, this interest never fully found its way into the 1.25-meter band.
The X-ray tube is a type of cathode-ray tube that generates X-rays when high voltage electrons hit the anode. [ 88 ] [ 89 ] Gyrotrons or vacuum masers, used to generate high-power millimeter band waves, are magnetic vacuum tubes in which a small relativistic effect, due to the high voltage, is used for bunching the electrons.
The RS-25 engine consists of pumps, valves, and other components working in concert to produce thrust. Fuel (liquid hydrogen) and oxidizer (liquid oxygen) from the Space Shuttle's external tank entered the orbiter at the umbilical disconnect valves and from there flowed through the orbiter's main propulsion system (MPS) feed lines; whereas in the Space Launch System (SLS), fuel and oxidizer ...
Alan Greenspan (born March 6, 1926) is an American economist who served as the 13th chairman of the Federal Reserve from 1987 to 2006. He worked as a private adviser and provided consulting for firms through his company, Greenspan Associates LLC.
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