SUPPLEMENT. 389 



appearances, is a consideration of the very first importance- 

 requiring, too, the employment of the finest object-glasses, and 

 that technical skill in the adjustment thereof, which is only 

 acquired by long practice ; and to this end the study of the dia- 

 t^macese will be found in the highest Degree advantageous. 

 Moreover, one must devote a good deal of study to this ghostly 

 parasite, which t:oo often infests, and to a greater or less extent 

 defeats the unsuspecting observer. And here again the little 

 diatom will render yeoman's service. 



Some months ago, Dr. Woodward, to whom we are all so 

 much indebted published in the London Monthly Microscopical 

 Journal, for the express purpose of displaying these illusory ap- 

 pearances, a lithograph from his photograph of Frmlulia, Sax- 

 onica. The objective used by Dr. Woodward was an American 

 l-18th, and was the same glass with which his inimitable photo- 

 graphs of the 19th Kobert band were accomplished. 



I took it for granted that the lithograph of Fmstulia Saxonica 

 was made with the glass intentionally placed out of adjustment, 

 and for the purpose of creating these particular lines. A copy 

 of the London journal is placed at our service this evening, and 

 I invite your inspection of the plate referred to. The whole 

 shell of the diatom will be seen badly distorted, the striae badly 

 denned, and the diffraction lines are immense I" * 



*I may here remark that only a few hours ago my attention was called to 

 a report of a meeting: of the London Microscopical Society, held April 5, 

 1876, and t eported in the May number of the London Microscopical Journal. 



Alluding: to the photographs of Frustulia Saxonica, Mr. John Mayall said: 

 " Kvery one who is familiar with the Frustul'a Saxonica, photographs of 

 which Dr. Woodward sent in illustration of his paper in December, knows 

 it to be one of the most difficult test-objects a diatom that ranks next to 

 Amphipleura pellucida. That particular form of Frustulia is one that I 

 have rare y seen resolved, except by lenses of the highest excellence. I 

 consider Dr. Woodward's photographs of it as in every way most remark- 

 able, evincing first-rate skill brought to bear on one of the finest known 

 lenses." It is hard to tell which is the most "remarkaole," the photographs, 

 or this statement of Mr. Mayall's. 



As there seems to be much difference of opinion as to these representa- 

 tions of Frustulia Saxonica, both at h me and abroad, a word or two may be 

 admissable. When the lithographs first made their appearance in the Lon- 

 don Monthly MicroscopiealJournal^ one of our most talented experts wrote 



