14G MANAGEMENT OF THE MICROSCOPE. 



nifying 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, an 

 Objective of perfect correction and adequate angle of aperture 

 will sustain this treatment with so little impairment in the perfec- 

 tion 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. In making any such comparisons, it must be constantly 

 borne in mind that the real question is, what can be seen ? It is 

 always desirable for the purposes of research to employ the loivest 

 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 

 different parts of the object be appreciated. Hence, in testing the 

 optical quality of a Microscope, the first question should be, not 

 what is its greatest magnifying power, but, what is the least mag- 

 nifying power under which it will show objects of a given degree 

 of difficulty. 



111. In making the Focal Adjustment, when low powers are 

 used, it will scarcely be necessary to employ any but the coarse 

 adjustment, or ' quick motion ; ' provided that the rack be well 

 cut, the pinion work in it smoothly and easily, without either 

 'spring,' 'loss of time,' or 'twist,' and the milled-head be large 

 enough to give the requisite leverage. All these are requisites 

 which should be found in every well-constructed instrument ; and 

 its possession of them should be tested, like its freedom from 

 vibration, by the use of high powers, since a really good coarse 

 adjustment should enable the observer to 'focus' an Objective of 

 l-8th inch with precision. What is meant by 'spring' is the 

 alteration which may often be observed to take place on the with- 

 drawal of the hand ; the object which has been brought precisely 

 into focus, and which so remains as long as the milled-head is 

 between the fingers, becoming indistinct when the milled-head is 

 let go. The source of this fault may lie either in the rack-movement 

 itself, or in the general framing of the instrument, which is so weak 

 as to allow of displacement by the mere weight or pressure of the 

 hand : should the latter be the case, the ' spring ' may be in great 

 degree prevented by carefully abstaining from bearing on the 

 milled-head, which should be simply rotated between the fingers. 

 By ' loss of time ' is meant the want of sufficient readiness in the 

 action of the pinion upon the rack, so that the milled-head may be 



