USE OF DIFFERENT EYE-PIECES. 161 



of a low power for a high one, great convenience is aftbrded by 

 Mr. Brooke's object-glass holder (§ 50). 



80. 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 pos- 

 sess a complete series Of objectives, he will generally find it best 

 to substitute on^ 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 amplification which they produce, by using each objective 

 first with the shallower, and then with the deeper eye piece. 

 Thus if a Microscope 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 (as is the case 

 with Messrs. Smith and Beck's new Educational Microscope, p. 

 103, note), such a range as the following may be obtained, — 55, 

 100, 200, 350 diameters ; or, with two objectives of somewhat 

 shorter focus, and with deeper eye-pieces (as is the case with an 

 instrument in the Author's possession, constructed by Kellner 

 of Wetzlar, whose Microscopes have acquired for themselves a 

 deservedly high reputation), — 88, 176, 350, 700 diameters. The 

 use of the "draw-tube" (§ 43) enables the Microscopist still fur- 

 ther 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 aperture be small, the loss of light is an equally 

 serious drawback. On the other hand, a combination of perfect 

 constructioin and wide 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 object- 

 ive of much shorter focus combined with a shallow eye-piece.' 

 In making any such comparisons, it must be constantly borne 

 in mind that the real question is, what can he seen ? It is always 

 desirable for the purposes of research, to employ the lowest power 

 with which the details of structure can be clearly made out; 

 since, the lower the power, the less is the liability to error from 

 false appearances, and the better can the mutual relations of the 

 difierent parts of the object be appreciated. Hence in testing 

 the optical quality of a Microscope, the question should always 



' The 4-lOths object-glass of Messrs. Smith and Beek was specially distinguished by 

 the Jurors of the Great Exhibition, as affording, by the use of deep eye-pieces and the 

 draw tube, a power fully equivalent in the resolution of difficult tests, to that which, a 

 few years previously, could only have been given by an objective of l-8th inch. 



11 



