220 ON THE THEORY OF THE MICROSCOPE. 



importance, such as increased field, &c., &c. In view of these 

 results, special significance must be attached to the analytical 

 limit here assigned to the respective functions of objective and 

 ocular, and to the constructive diagram founded thereon. Every 

 fault of the image-forming process which influences in a general 

 sense the delineation, finds its full expression at once in the 

 quality of the virtual image of the object which is produced by 

 the front lenses of the objective acting as a simple magnifying 

 lens. In contrast with this the whole ocular apparatus, consisting 

 of tube and lenses, acting as a telescope, merely plays the part of 

 an indiff'erent magnifying mechanism, which serves simply to 

 spread out before the eye at the necessary visual angle the image 

 formed by the objective without adding to, or taking anything 

 from the quality of its contents. These contents, so far as 

 possible details are concerned, will depend upon the angular mag- 

 nitude of the dispersion circles which are formed in the objective 

 virtual image where there ought to be sharply defined points 

 only, and which are the result of faults of the image-forming 

 process inherent in the construction of the objective. Taking 

 their intervention in the final effect into account, there will be 

 found for every objective a particular angular amplification 

 obtainable at will by means of length of tube and strength of 

 ocular, which must exactly suffice to enable any eye, possessing 

 normal capacity of vision, to recognize perfectly aU the details 

 that can possibly be delineated in the virtual image formed by the 

 objective. And this, which may be termed ** necessary angular 

 amplification," may be looked upon as the measure of the 

 relative perfection of the objective. From it maybe determined, 

 by easily deducible rules, with the help of its focal length, the 

 necessary linear amplification, i.e., the numerically estimated 

 amount of magnification at which the capacity of performance 

 of the objective is exhausted. This is then, the lowest magnify- 

 ing power, with which every detail will be seen that can be 

 delineated by it, according to its measure of optical power. A 

 further amplification may still be serviceable, as the detail will 



