The elements of astronomy; a textbook . etween two objects beingmade by superposing the image of oneof them upon that of the other. In usingthe filar-micrometer we have to look two ways at once to be sure thateach of the two wires accurately bisectsthe star upon which it is set: with a mx M0 M2 double-image micrometer, the observers *Si ® ® O attention is concentrated upon a single °he without distraction. The heliometer is a complete telescope, equatorially mounted,and having its object-glass (usually from four to six inches indiameter) divided along its diameter, a


The elements of astronomy; a textbook . etween two objects beingmade by superposing the image of oneof them upon that of the other. In usingthe filar-micrometer we have to look two ways at once to be sure thateach of the two wires accurately bisectsthe star upon which it is set: with a mx M0 M2 double-image micrometer, the observers *Si ® ® O attention is concentrated upon a single °he without distraction. The heliometer is a complete telescope, equatorially mounted,and having its object-glass (usually from four to six inches indiameter) divided along its diameter, as shown in Fig. semi-lenses are so mounted that they can slide past eachother for a distance of three or four inches, the distance beingaccurately measured by a delicate scale, which is read by along microscope that comes down through the telescope-tubeto the eye end. The tube is mounted in such a way that itcan be turned around in its cradle, so as to make the line ofdivision of the lenses lie at any desired position 414 APPENDIX. [$543 When the two halves of the object-glass are so placed thattheir optical centres, 0 and 1 or 0 and 2, coincide, they act as asingle lens, and form but a single image for each object in thefield of view: but as soon as they are separated, each half-lensforms its own image. The distance between any two objectsin the field of view is measured by making their images co-incide, as indicated in the lower part of the figure, where MQand S0 are the images of Mars and of a star, formed by thestationary half, B, of the object-glass, which has its centreat 0. M1 and Sx are the images formed by the other halflens, A, when its centre is 1, and M2 and S2 are the images whenits centre is 2. The distance between the images of S andM is therefore either 0 1 or 0 2, read off on the sliding direction in which the line 10 2 has to be set to effect thecoincidence, gives the direction, or position angle, fromilf to S. 544. The Transit Ins


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