Elements of natural philosophy (Volume 2-3) . on (40).have we Same in adifferent form; A 1 1 A (F) 1 1 1 ] f Fu ELEMENTS OF OPTICS. 253 or, writing D for _; and representing, as before, the Jpowers of the field and eye lenses by L and I, A D- V Final value formagnifyingpower: from which it is obvious that the magnifying power maybe varied to any extent by properly regulating the po-May be varied;sition of the object; but a change in the position of theobject would require a change in the position of the eye-glass, and two adjustments would, therefore, be neces-sary, which would be inconvenient
Elements of natural philosophy (Volume 2-3) . on (40).have we Same in adifferent form; A 1 1 A (F) 1 1 1 ] f Fu ELEMENTS OF OPTICS. 253 or, writing D for _; and representing, as before, the Jpowers of the field and eye lenses by L and I, A D- V Final value formagnifyingpower: from which it is obvious that the magnifying power maybe varied to any extent by properly regulating the po-May be varied;sition of the object; but a change in the position of theobject would require a change in the position of the eye-glass, and two adjustments would, therefore, be neces-sary, which would be inconvenient. For this reason, itis usual to leave the distance between the lenses unal-tered and to vary only the distance of the object to suitdistinct vision. It is, however, convenient to have thepower of changing the distance between the glasses, asby that a choice of magnifying powers between certainlimits may be obtained, and for this purpose the object glassesinand eye glasses are set in different tubes. different tubes. Usual practice; Fig. Reflectingtelescope; § 82. If the field lens of the astronomical telescope bereplaced by a field reflector IIN, whose optical centreis at (7, as indicated in the figure, we have the commonastronomical reflecting telescope. C being the opticalcentre, d becomes equal to/ - {F„\ and Equation (70),becomes, by first changing the sign of /, and then sub- Explanation;stituting this value for d> 25i NATURAL PHILOSOPHY. Magnifyingpower forterrestrialobjects; Same forcelestial objects. AA
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