REGULATION OF TRANSMITTED LIGHT. 171 



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. 



87. The equable illumination of the entire field having been 

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

 gulated by the Diaphragm-plate (§ 55). This must depend very 

 much upon the nature of the object, and upon the intensity of 

 the light. Generally speaking, th^ 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 oilia, — those minute vibratile filaments, with 

 which almost every Animal is provided in some part of its or- 

 ganism, and which many of the humbler Plants also possess, — 

 can only be discerned in many instances, when the light is ad- 

 mitted 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 raysthrough it, great care should be taken to pro- 

 tect it entirely from incident light; since this extremity weakens 

 the effect of that which is received into the microscope by trans- 

 mission. It is by daylight that this interference is most likely 

 to occur : since, if the precautions already given (§ 76) 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 vapor ; 

 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. ITothing 

 more is necessary for its permanent avoidance, than the inter- 

 position 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 interferes with access to the left side of the stage'; and the 

 interposition 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 by daylight, is recommended never to omit 



