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ainong inicroscopists undoubtedly arises from the fact that, as 

 we have noticed in speaking of the cause of the visibility of 

 objects in general, outside of the microscojnc field it really is 

 immaterial, in so far as the mere visibility is concerned, and 

 makes not the least difference whether your illuminating 

 pencil is convergent, parallel, or divergent (an augmentation 

 of the intensity in each case, however, being necessary) ; and 

 persons having become accustomed to the one or the other,liave 

 taught and given out to the world that such was the true 

 mode of illumination. Those advocating the use of either 

 as on trial may be found expedient, testify by their indecision, 

 I regret to say, to a Avant of that nicety of discernment, so 

 eminently needed in microscoj^ic research. 



Andjfirstly, of central or direct illumination. That a slightly 

 convergent pencil of light, as e. g. obtained from the concave 

 mirror, is the one most suitable for central or direct illumina- 

 tion, is a truth that I think both theory and trial will fully 

 support. That such is the case arises from the fact that, by a 

 convergent pencil, we are enabled to condense a greater amount 

 of light upon the object, and so give greater sharpness and dis- 

 tinctness to the impressions made upon the retina. And, again, 

 in a pencil of rays thrown upon an object, a part only are 

 irregularly reflected, thereby rendering the object visible, the 

 other, and frequently a major part, passing by and around 

 the object, and falling upon the surface of tlie objective, and 

 then and there becoming obedient to all the laws of refrac- 

 tion the same as those emanating from the object itself. Now, 

 the course of a convergent pencil of light in its refractions, 

 while passing through an objective, differ widely from that 

 of the divergent pencils emanating from the object — of course 

 much more so than that of parallel rays — and consequently 

 the dimness, obscuration, and want of definition resultant 

 from interference, and from other rays entering the eye 

 besides those emanating from the object, is more or less 

 removed. It is with reference to this very point that we are 

 instructed that our pencil of rays should, in as far as possible, 

 be only of a size equal to the illumination of the object; and 

 no greater proof can be had of the interference caused by 

 rays thus entering the objective, other than those emanating 

 from the object, than the fiir more excellent definition ob- 

 served on the interposition of a diaphragm in close apposition 

 to the slide, with an opening only sufficiently large for the 

 illumination of the object. A series of carefully conducted 

 experiments, and an impartial judgment based thereon, 

 also prove the choice of a moderately convergent pencil 

 for central illumination to be incontrovertiblv correct. 



