Elements of natural philosophy (Volume 2-3) . engths ofthe first and second lenses. Multiplying this by Equation(75), member by member, we have for the magnifyingpower of the terrestrial telescope, A! 1L VI Magnifyingpowir of theterrestrial telescope. 252 NATURAL PHILOSOPHY. objects appear And since the ratio of A to A! is positive, objects will erect. appear through this instrument erect. Compoundmicroscope; § 81. If, now, \a\q, object approach the field lens, /in Equation (71), will increase, and the magnifying powerbecome proportionably greater; but this wxmld requirethe tube containing the
Elements of natural philosophy (Volume 2-3) . engths ofthe first and second lenses. Multiplying this by Equation(75), member by member, we have for the magnifyingpower of the terrestrial telescope, A! 1L VI Magnifyingpowir of theterrestrial telescope. 252 NATURAL PHILOSOPHY. objects appear And since the ratio of A to A! is positive, objects will erect. appear through this instrument erect. Compoundmicroscope; § 81. If, now, \a\q, object approach the field lens, /in Equation (71), will increase, and the magnifying powerbecome proportionably greater; but this wxmld requirethe tube containing the eye lens to be drawn out to ob-tain distinct vision, and to an extent much beyond thelimits of convenience if the object were very near. Thisdifficulty is avoided by increasing the power of the ob-ject lens, as is obvious from Equation (54); and whenthis is carried to the extent required for very great prox-imity, the instrument becomes a compound microscope,which is employed to examine minute objects. The com- Compoundmicroscope; Fig. same in principle pound microscope not differing in principle from the as refracting i_ -i ., . p . I r teiescopo; telescope, its magnifying power is given by the Equa- Its magnifyingpower; tion, 4 = -11 = A m m r and substituting for -_ its value in Equation (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
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