4l6 MANIPULATION AND PKESERVATION OF THE MICROSCOPE 



account be altered, and it is understood that the Limp was adjusted 

 to the picture A in fig. 344 by inspection and without the micro- 

 scope. A very slight movement in azimuth, however, is enough to 

 effect the desired end (fig. 364), and all that now remains is to open 

 the full aperture of the condenser and put in the smallest stop ; if 

 this does not stop out all the light, a larger one must be tried ; but 

 it is of the greatest importance that the smallest stop possible be 

 used, a very little difference in the size of the stop making a remark- 

 able difference in the quality of the picture. Hence the need of a 

 large and varied supply of stops with all condensers. 



On account of some residual spherical aberration the condenser 

 will probably have to be racked up slightly to obtain the greatest 

 intensity of light. 



In fig. 364 the expanded edge of the flame covers the triceratium. 

 When the whole aperture of the condenser is opened the size of that 

 disc will not be altered, its intensity only will be 

 increased. When the stop is placed at the back 

 of the condenser, only in that part of the field 

 represented by the disc of light will the object 

 be illuminated on a dark ground. If, therefore, 

 the disc of light does not cover the object or ob- 

 jects, bring the lamp nearer the mirror. The 

 size of the disc of light depends on three things : 

 FIG. 364. o. The diameter of the bull's-eye. 



ft. The length of the path of the rays from 

 the bull's-eye to the sub-stage condenser. 



y. The magnifying power of the condenser. 



If a and y are constants, the only way of varying the size of the 

 dark field is by ft. 



In the same way the intensity of the light in the disc depends on 

 three things. 



A. The initial intensity of the illumination. 



B. The angular aperture of the bull's-eye. 



C. The angular aperture of the sub-stage condenser. 



If the student will thoroughly and practically understand the 

 above series of single demonstrations, and ponder such inevitable 

 variations as practice will bring in regard to them, the ' difficulties 

 of illumination ' will have practically passed away. 



There are two kinds of microscopical work one, the more usual 

 and comparatively easy, is the examination of an object to see some- 

 thing which is known. The other is the examination of an object 

 in search of the unknown. Thus some blood may be examined 

 for the purpose of finding a white corpuscle. It matters little 

 what is the quality of either the lens or the illumination or the 

 microscope, or whether the room is darkened or not, because the 

 observer knows that there is such a thing as a white corpuscle. It 

 is quite immaterial as to whether the observer had ever seen one or 

 not ; so long as he possesses the knowledge that there is such a thing, 

 the finding of it, even under unfavourable conditions, will be an 

 easy task. 



But if the observer has not that knowledge, he may examine 



