156 



This mode, he informs us, consists in the use of a prism, rect- 

 angular or equilateral, in place of the mirror ; and he ascrihes 

 the superiority of the results which are thus obtained to the 

 use of parallel rays thereby thrown upon the object. 



I have used Mr. Reade's prism, and can bear witness to 

 its superiority over mirror, achromatic condenser, or kettle- 

 drum illuminator, but do not agree at all Avith him as to the 

 cause of the result ; on the contrary, both reason and trial 

 demonstrate most markedly to the unprejudiced observer that, 

 by the addition of a lense to said prism, constituting at once 

 the achromatic lenticular prism (as recommended above when 

 treating of centric illumination) we gain most decidedly and 

 then really seemed to have at last reached Avhat may be 

 novv truly termed the very perfection of microscopical illu- 

 mination. 



In returning to the consideration of the shadows of objects, 

 as cast by oblique light, we can at once see the reason 

 for this. Although, as previously stated, the penumbra or 

 lesser shadow, interfering in proportion to its extent with 

 the umbra or true shadoAV, is less when parallel rays impinge 

 upon an object, than when convergent rays are thrown 

 thereupon ; still, it will be found by experiment outside 

 the microscope, that a moderately convergent pencil of light, 

 owing, in a great measure, undoubtedly, to the greater intensity 

 of illumination, will cast a blacker, sharper, and better-defined 

 shadow, though somewhat smaller than is obtained when 

 a parallel pencil is used to effect the same ; and identically 

 the same result is present when used in microscopical illu- 

 mination. It may be said that I alone am able to see this; 

 but the testimony of many able, experienced, and well-known 

 microscopists bear me out in my assertion. 



It is but recently that I had the pleasure of exhibiting this 

 mode of illumination to such men as Messrs. Frey and 

 Mason, members of our society; and still more recently at 

 a reunion at my house, to Mr. Ward, Drs. Eich and Allen, 

 and Professor Edwards, all members of the American Micro- 

 scopical Society of the City of New York ; and by the 

 additional presence of Professor Hamilton Smith, of Ohio, 

 the knowledge of whose skill, experience, and judgment is 

 not limited to the microscopists of this country alone, but is 

 equally known and accredited upon the other side of the 

 water, I felt yet more strongly that the truth, either pro or 

 con, as it might happen, still that the truth, and nothing but 

 the truth, would be elicited. It is, I say, to such men that 

 I have had the good fortune of exhibiting this or, as Mr. 

 Reade would say, " my mode of illumination," and the 



