ARRANGEMENT FOR TRANSPARENT OBJECTS. 155 



judged of by its formation of images of window-bars, cbimneys, 

 &c, upon any semi-transparent medium placed in tbe plane of the 

 object. It is only, however, when 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 wonld not in 

 this mode be equally 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 recom- 

 mended (§ 78), have had its larger aperture filled with such L a 

 diffusive 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 require, save when Erectors (§§ 64, 

 65) are 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 

 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 so doing, the light may be withdrawn from some part previously 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 altera- 

 tion in the distance of the Mirror from the stage ; or, from tbe 

 very oblique position of the Mirror, the cone is too much narrowed 

 across one of its diameters, 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, as;ain, 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 (in the day-time) by 



