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A STUDY has been made of materials available and suitable for the construction of transparent windows employed for observation inside the combustion-chamber. Materials that can be used include quartz, spinel, sapphire, periclase, and fluorite. The transparency of these materials in the infrared region increases in the order just mentioned. Mechanical strength varies greatly, being highest for sapphire and lowest for fluorite. Resistance to the chemical action of the combustion products varies widely. Sapphire is not attacked by any products of combustion, including the lead oxide present when leaded fuels are used. The choice of a suitable material for a given investigation will be based on a judicious balance of the four chief characteristics: mechanical resistance, chemical resistance, transparency, and cost.