A secondary optical system for object navigation in an array of beacons is provided that includes an optical source having at least one optical emitter emitting an optical signal and that is mounted to either the moving object or a beacon of the array of beacons. A method of preparing a reaction container comprises preparing a graphite mixture by mixing first and second graphite powders having particle sizes different from each other, preparing a graphite molded body by pressing the graphite mixture, and processing the graphite molded body. Therefore, a large bright virtual image with reduced aberration can be displayed. Further, a method for obtaining the electrocardiogram using two electrodes is also provided. In additional embodiments, the platform seal assembly can have two or three buffer cavities formed between additional restrictions. A profile of at least a second portion of the structure on the wafer is obtained using a second metrology apparatus.