CHOICE OF MAGNIFYING POWER. 145 



determined by the character of the object. Large objects presenting 

 no minute structural features should always be examined in the 

 lirst instance by the lowest powers, whereby a general view of their 

 nature is obtained ; and since, with lenses of comparatively long 

 focus and small angle of aperture, the precision of the focal 

 adjustment is not of so much consequence as it is with the higher 

 powers, not only those parts can be seen which are exactly in 

 focus, but those also can be tolerably well distinguished which 

 are not precisely in that plane, but are a little nearer or more 

 remote. "When the general aspect of an object has been sufficiently 

 examined through low powers, its details may be scrutinized under 

 a higher amplification ; and this will be required in the first 

 instance, if the object be so minute that little or nothing can be 

 made out respecting it save when a very enlarged image is formed. 

 The power needed in each particular case can only be learned by 

 experience ; that which is most suitable for the several classes of 

 objects hereafter to be described will be specified under each head. 

 In the general examination of the larger class of objects, the 

 range of power that is afforded by the Erector in combination with 

 the Draw-tube (§§ 63,64) will often be found useful ; whilst for 

 the ready exchange of a low power for a higher one, great con- 

 venience is afforded by the Nose-piece (§ 73). 



110. "When the Microscopist wishes to augment his magnifying 

 power, he has a choice between the employment of an Objective of 

 shorter focus and the use of a deeper Eye-piece. If he possess a 

 complete series of Objectives, he will generally find it best to sub- 

 stitute one of these for another without changing the Eye-piece for 

 a deeper one ; but if his ' powers ' be separated by wide intervals, 

 he will be able to break the abruptness of the increase in amplifi- 

 cation which they produce, by using each Objective first with the 

 shallower and then with the deeper Eye-piece. Thus if a Micro- 

 scope be only provided with two Objectives, of 1 inch and l-4th 

 inch focus respectively, and with two Eye-pieces, one nearly double 

 the power of the other, such a range as the following may be 

 obtained, — 60, 90, 240, 360 diameters ; or, with two Objectives 

 of somewhat shorter focus, and with deeper Eye-pieces (as in some 

 French and German instruments), — 88, 176, 350, 700 diameters. 

 In the examination of large Opaque objects having uneven surfaces, 

 it is generally preferable to increase the power by the Eye-piece 

 rather than by the Objective ; thus a more satisfactory view of 

 such objects may usually be obtained with a 3-inch or 2-inch 

 Objective and the b Eye-piece, than with a 1 4 -inch or 1-inch 

 Objective and the a Eye-piece. The reason of this is, that in 

 virtue of their smaller Angle of Aperture, the Objectives first named 

 have a much greater amount of 'penetrating power' or 'focal 

 depth' than the latter (§ 131, i.) ; and that in the case just specified 

 this quality is of the first importance. The use of the Draw- 

 tube (§63) enables the Microscopist still further to vary the mag- 



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