157 



testimony of each and all has been outspoken at once freely 

 in its favour. 



Professor Smithy on viewing the resolution of the SurireUa 

 gemma, with a -^ immersion made for me by Mr. William 

 Wales, of this citj^, had no hesitation in acknowledging the 

 great beauty and superiority of the same ; saying that he had 

 never seen it better. When we take into consideration that, 

 with Hartnach of Paris, with Lobb of London, and various 

 other foreign celebrities, Professor Smith has critically viewed 

 this same shell, as yet the most difficult of test objects ; (for 

 Professor Smith denies a true and indisputable lining to 

 Amjjhipleura peUucida, although these very men attempted 

 to show it to him) ; and then hear him tell ns that the 

 exhibition of this shell, as shown by the -^ immersion was 

 superior to anything he had yet seen — can we wonder for a 

 moment at his additional remark that it must be due, more 

 or less, to this mode of illumination ? 



A -jL immersion of ]Mr. Wales's make, the jjroperty of Dr. 

 Rich, was also exhibited ; and although it was impossible, by 

 reason of not having an object provided with a cover of the 

 necessary thinness, to judge fully of its merits, still it was 

 observed that the quantity of light furnished by this simple 

 mode of illumination was amply sufficient. 



A recapitulation is hardly requisite, as the inference is 

 probably already drawn that for oblique or eccentric illumi- 

 nation a moderately convergent pencil of rays is that best 

 adapted to the purpose, and that the means of obtaining such 

 pencil is through the agency of the same achromatic lenticu- 

 lar prism, as recommended, and described Avhen speaking of 

 central or direct illumination. To those provided therewith 

 and desirous of trying for themselves the effect of parallel 

 rays, and not wishing to purchase a separate prism for such 

 trial, I would state that, by placing the lamp or source of 

 illumination at a distance from the lens, equal only to the 

 focal distance of said lens, the emergent rays become parallel, 

 and may be used, especially with low power, wdth great satis- 

 faction and pleasure. The intensity or amount of illumination 

 thus obtained is far greater than that received from the 

 plane mirror being even sufficient for a -yL-. 



It may not be amiss, in conclusion, to consider briefly the 

 source of illumination itself. The St. Germain student-lamp 

 is, 7J«r excellence, the lamp of the microscopist. The chimney 

 of this lamp should be encircled with an external metallic 

 tube, having, at the point coinciding Avith the constriction of 

 the glass chimney, two openings placed opposite to each other. 

 These openings should be one and a quarter inch in diameter. 



