ON THE ILLUMINATION OF OBJECTS. 



193 



The prism certainly has some advantages over a mirror ia 

 bringing out minute markings, and also in reflecting more 

 light. 



Mr. Varley's dark chamber has been already described at 

 page 112: it is very useful where an achromatic condenser is 

 not applied, as the light, having no lens to pass through, 

 is not decomposed ; in this consists its value. The method of 

 using it being similar to that of the achromatic or Wol- 

 laston's condenser, a separate description is not required in 

 this place. 



Direct Light. — Many very dehcate objects may be seen to 

 the greatest advantage by what is termed direct light, which 

 is obtained readily by the removal of the mirror, and by 

 placing a lamp or candle behind the stage in its stead : for this 

 purpose it will be necessary to incline the microscope, so that 

 the rays may pass through the stage, and in order that the 

 posture in which the observer must be placed be not ren- 

 dered fatiguing, it is the best plan at first so to raise the 

 instrument from the table, that the eye-piece may be on a 

 level with the eye. In the day tune, the light taken directly 

 from a white cloud opposite the sun wiU serve to bring out 

 some objects very beautifully, but it is impossible to give 

 a list of those that are best shown by direct hght, this can 

 only be known after very extensive practice, as the efiects 

 wiU vary with each different magnifying power. 



Fig. 122. 



Fig. 122 represents the method of illuminating by direct 

 light. a is the compound body, placed in a horizontal 

 13 



