156 MANAGEMENT OF THE MICROSCOPE. 



the images of 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 dissipated in the same way, the lamp 

 should be brought a little nearer. 



117. The equable illumination of the entire field having been 

 thus obtained, the quantity of light to be admitted should be regu- 

 lated by the Diaphragm-plate (§ 78). 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 — 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 protect 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 inter- 

 ference is most likely to occur ; since, if the precautions already 

 given (§ 106) 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 produced, 

 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 vapour ; and he may at once satisfy himself of the cause by 

 interposing his hand between the stage and the source of light, 

 when the immediate increase of brilliancy and of distinctness will 

 reveal to him the occasion of the previous deficiency in both. 

 Nothing more is necessary for its permanent avoidance than the 

 interposition of an opaque screen (blackened on the side towards 

 the stage) between the window and the object ; care being of course 

 taken that the screen does not interfere with the passage of light 

 to the Mirror. Such a screen may be easily shaped and adapted 

 either to be carried by the stage itself, or by the stand for the 

 condenser ; but it is seldom employed by Microscopists, as it inter- 

 feres with access to the left side of the stage ; and the interposi- 

 tion of the hand, so often as it may be needed, is more frequently 

 had recourse to in preference, as the more convenient expedient. 

 The young Microscopist who may be examining transparent objects 



