On the Navicula Spencerii. 265 





Art. XXVI. 



e remarks on the Navicula & 

 difficult test object ; by J. W 



I have learned from several quarters, that the notice of the new 

 test object to which I gave the name Navicula Spencerii contain- 

 ed in Quekett's Practical Treatise on the Microscope recently 

 published, has given rise to most erroneous impressions which 

 justice to all concerned requires me to correct as far as is in my 

 power. The work alluded to has not yet reached me, but I am 

 indebted to a friend for the 8th and 9th plates, and the following 

 extract referring to the test in question. 



Navicula Spencerii — Early in the present year, Mr. Matthew Marshall re- 

 ceived some specimens of this species from Professor Bailey, of West Point, 

 New York, who stated that an object-glass, constructed by a young artist of 

 the name of Spencer, living in the back woods, had shown three sets of lines 

 on it, when oth^r glasses of equal power, made by the first English opticans, 

 had entirely failed to define them. Mr. Marshall was supplied with the iden- 

 tical specimens on which Mr. Spencer's object-glass had been tried ; these 

 have since been carefully examined by Mr. Marshall and Mr. Warren De La 

 Rue, and the nature of the markings clearly made out. Mr. De La Rue, has obli- 

 gingly furnished the author with Plate IX, in which he has faithfully delineated 

 a specimen of N. Spencerii, as viewed under a power of H00 diameters, and a 

 portion of the same magnified 1,900 diameters, from which it will be plainly 

 seen that the lines discovered by Mr. Spencer are in reality dots, and arranged 

 so as to exhibit both transverse, longitudinal, and even oblique stria*, when 

 viewed by an object-glass not capable of separating the dots one from the 

 other. Air. De La Rue has further made out that the dots are not projections 

 from the surface, but are either perforations or depressions. The shape of the 

 shell is not unlike that of a small kind of N. Hippocampus, which the markings 

 also very much resemble.— p. 440. 



The above notice is accompanied by a plate drawn by Mr. W. 

 De La Rue representing the object as seen by him, magnified 800 

 diameters, and a figure of a portion of the same magnified 1900 

 diameters is also given to show the supposed perforations. 



I believe the impressions which have been generally received 

 by American microscopists, on the perusal of the above, are that 

 it indirectly charges me 1st with underrating the English micro- 

 scopes ; 2d with over-rating both the merits of our own artist 

 Spencer, and the difficulties of the N. Spencerii as a test object, 

 and lastly, that the structure, or markings upon it, have been 

 wholly mistaken both by Spencer and myself in consequence of 

 0l *r working with glasses "not capable" of properly resolving 

 the object in question, and hence greatly inferior to those made 

 in London. A few remarks upon each of these points will I 

 trust suffice to correct these errors. To the excellence of the 

 lenses made by Ross and Powell of London, I have always given 



most willing testimony, and in the only allusion which I have 



Second Series, Vol. VII, No. 20.— March, 1349. 31 



