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ability and attainments hold most opposite opinions. In 

 endeavouring to ascertain what are really the facts in the 

 case, and by what rule or rules we should be guided, it 

 becomes necessary, laying aside all previous bias or hitherto 

 conceived opinions, to commence de novo with first prin- 

 ciples, and guided by known and absolute laws of light and 

 vision, in conjunction with a careful, complete, and unwearied 

 series of experiments, Avork out the much desired end. 



Rays of light impinging upon an object, it is well known, are 

 disposed of either, first, by regular reflection, as it is termed, in 

 W'hich the angles of incidence and reflection are the same, and 

 which reflection, bear in mind, in no wise renders such object 

 visible ; secondly, by absorption, a manner of disposal not con- 

 cerning the microscopist ; or, thirdly, by irregular reflection, in 

 which each superficial atom of the object becomes itself a focal 

 point from which pencils of light diverge in every direction, 

 and which reflection is totally independent of the form or angle 

 of incidence of the bundle of rays falling upon such object. 

 In truth, the particles or molecules of matter composing the 

 surface of an object become original centres, fi-om which light 

 emanates and is reflected, or, to use here a term far more 

 proper, is radiated in every direction. It is by means of this 

 irregular reflection — a reflection, observe, governed by no law 

 of incidence — a reflection, in fact, in which the object becomes 

 a new sun or centre — it is bv this reflection (unhappily 

 termed irregular reflection), and by this alone, that objects 

 are rendered visible to us. Such visibility (laying aside the 

 varied capability of diverse objects for such reflection) de- 

 pends then solely and entirely upon the reception by the 

 object, from some original source or centre of light, of a 

 suflicient quantity of rays, and except in the intensity of illu- 

 mination, which will diminish in equal ratio as the size of 

 the angle of obliquity of such surface to the direction of the 

 rays is diminished, is totally independent of the angle of 

 incidence of such rays, or of their parallellism, convergence, 

 or divergence. 



In the microscopic observation of transparent objects we 

 have at our disposal and make use of tAvo principal modes 

 of illumination — the one central or direct, in which the illu- 

 minating pencil is a continuation of the optical axis of the 

 instrument — the other oblique or eccentric, in which the 

 pencil of rays forms an angle of greater or less degree with 

 such optical axis. Various microscopists, writing upon the 

 subject, advise, some, the use of parallel rays ; others, the use 

 of convergent rays, and, again, others the use of the one or 

 the other, as on trial answers best. This variance of opinion 



