170 MANAGEMENT OF THE MICROSCOPE. 



small objects are being viewed under high magnifying powers, 

 that such a concentration of the light reflected by the mirror is 

 either necessary or desirable ; for with large objects, seen under 

 low powers, the field would not in this mode be equably 

 illuminated. The diffusion of the light over a larger area may 

 be secured, either by shifting the mirror so much above or so 

 much below its previous position, that the pencil will fall upon 

 the object whilst still converging or after it has met and 

 diverged ; or, on the other hand, by the interposition of a plate 

 of ground glass in the course of the converging pencil, — ^this 

 last method, which is peculiarly appropriate to lamp-light, being 

 very easily had recourse to, if the diaphragm plate, as formerly 

 recommended (§ 55), have had its largest aperture filled with 

 such a difliised medium. The eye being now applied to the 

 Eye-piece, and the body being "focussed," the object is to be 

 brought into the exact position required, by the use of the 

 traversing movement, if the stage be provided with it ; if not, 

 by the use of the two hands, one moving the object-slide from 

 side to side, the other pushing the ledge, fork, or holder that 

 carries it, either forwards or backwards, as may be required. It 

 is always to be remembered, in making such adjustments by the 

 direct use of the hands, that, owing to the inverting action of 

 the microscope, the motion to be given to the object, whether 

 lateral or vertical, must be precisely opposed to that which 

 its image seems to I'equire, save when the Erector (§ 44) is 

 employed. When the object has been thus brought fully into 

 view, the Mirror may require a more accurate adjustment. 

 What should be aimed at, is the diffusion of a clear and equable 

 light over the entire field; and the observer should not be 

 satisfied, until he has attained this object. If the field should 

 be darker on the one side than on the other, the mirror should 

 be slightly turned in such a direction as to throw more light 

 upon that side; perhaps in so doing, the light maybe withdrawn 

 from some part previously illuminated; and it may thus be 

 found that the pencil is not large enough to light up the entire 

 field. This may be owing to one of three causes : either the cone 

 of rays may be received by the object too near to its focal apex, 

 the remedy for which lies in an alteration in the distance of the 

 mirror from the stage ; or, from the very oblique position of the 

 mirror, the cone is too much narrowed across one of its diame- 

 ters, and the remedy must be sought in a change in the position 

 either of the microscope or of the lamp, so that the face of the 

 mirror may not be turned so much away from the axis of vision; 

 or, again, from the centre of the mirror being out of the optical 

 axis of the instrument, the illuminating cone is projected 

 obliquely, an error which can be rectified without the least 

 difficulty. If the cone of rays should come to a focus in the 

 object, the field is not unlikely to be crossed by the images of 

 window-bars or chimneys,' or the form of the lamp-flame may be 



