136 THE MICROSCOPE AND ITS REVELATIONS. 



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 ob- 

 jects 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 Zeiss's Adjustable Low-power Objective ( 159, I.) will 

 often be found useful; whilst for the ready exchange of a low power for a 

 higher one, great convenience is afforded by the Nose-piece ( 96). 



136. 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 frequently find it best to substitute one of these for 

 another without changing the Eye-piece for a deeper one; but if his 

 6 powers 'be separated by wide intervals, he will be able to break the 

 abruptness of the increase in amplification which they produce, by using 

 each Objective first with the shallower and then with the deeper Eye- 

 piece. Thus, if a Microscope be provided only with two Objectives of 

 1-inch and l-4th inch focus respectively, and with two Eye-pieces, OIIQ 

 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 exami- 

 nation 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 IJ-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 ( 158, i.); and in the case just specified 

 this quality is of the first importance. The use of the Draw- tube ( 83> 

 enables the Microscopist still further to vary the magnifying power of his 

 instrument, and thus to obtain almost any exact number of diameters he 

 may desire, within the limits to which he is restricted by the focal length 

 of his Objectives. The advantage to be derived, however, either from 

 'deep Eye-piecing' or from the use of the Draw-tube, will mainly depend 

 upon the quality of the Object-glass. For, if it be imperfectly corrected, 

 its errors are so much exaggerated, that more is lost in definition than is 

 gained in amplification; whilst, if its apertures be small, the loss of light 

 is an equally serious drawback. On the other hand, an Objective of 

 perfect correction and adequate angle of aperture will sustain this treat- 

 ment with so little impairment in the perfection of its image, that a 

 magnifying power may be obtained by its use, such as, with an inferior 

 instrument, can only be derived from an Objective of much shorter 

 focus combined with a shallow Eye-piece. The author thinks it a great 

 mistake, however, to attempt to make an Objective of medium power 

 ordinarily do the work on which an Objective of high power should 

 properly be employed. For not only can it not be brought up to this. 



