THE MICROSCOPE AND ITS REVELATIONS. 



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 end. 

 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 Jight 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 that side; perhaps in so doing, the light may be withdrawn from some 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 alteration in the dis- 

 tance 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 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, again, from the centre 

 of the Mirror being out of the optic axis of the instrument, so that 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 

 window-bars or chimneys, or (at night) the form of the lamp-flame may 

 be distinguished upon it; the former must be got rid of by a slight 

 change in the inclination of the Mirror; and if the latter cannot be dis- 

 sipated in the same way, the lamp should be brought a little nearer. 



143. The equable illumination of the entire field having been thus 

 obtained, the quantity of light to be admitted should be regulated by the 

 Diaphragm-plate ( 98). This must depend very much upon the nature 

 of the object, and upon the intensity of the light. Generally speaking, 

 the more transparent the object, the less light does it need for its most 

 perfect display; and its most delicate markings are frequently only made 

 visible, when the major part of the cone of rays has been cut off. Thus 

 the movement of the cilia those minute vibratile filaments with which 

 almost every Animal is provided in some part of its organism, and which 

 many of the humbler Plants also possess in the early stages of their exist- 

 ence can only be discerned in many instances when the light is admitted 

 through the smallest aperture. On the other hand, the less transparent 

 objects usually require the stronger illumination which is afforded by a 

 wider cone of rays; and there are some (such as semi-transparent sections 

 of Fossil Teeth) which, even when viewed with low powers, are better 

 seen with the intenser light afforded by the Achromatic Condenser. 

 In every case in which the object presents any considerable obstruction 

 to the passage of the rays through it, great care should be taken to pro- 

 tect it entirely from incident light; since this extremely weakens the 

 effect of that which is received into the Microscope by transmission. It 

 is by daylight that this interference is most likely to occur; since, if the 

 precautions already given ( 132) respecting the use of lamp-light be 

 observed, no great amount of light can fall upon the upper surface of the 

 object. The observer will be warned that such an effect is being pro- 

 duced, by perceiving that there is a want not only of brightness but of 

 clearness in the image, the field being veiled, as it were, by a kind of 

 thin vapor; and he may at once satisfy himself of the cause, by inter- 

 posing his hand between the stage and the source of light, when the 

 immediate increase of brilliancy and distinctness will reveal to him the 

 source of the previous deficiency in both. Nothing more is necessary for 

 its permanent avoidance, than the interposition of an opaque screen 



