A. A. C. ELIOT MERLIN ON DIATOM STRUCTURE. 579 



blue-bottles." We may thus assume that diatoms had not at 

 that period appeared on the scene to trouble the optician ' ' at 

 the Sign of Tycho Brahe's Head, No. 60, in Fleet Street, London." 

 Then who was the first man to dot the first diatom ? * Possibly 

 Dr. Goring, who, " is said to have discovered that the structure 

 of certain bodies could be readily seen in some microscopes 

 and not in others. These bodies he named test-objects ; he then 

 examined these tests with the achromatic combination before 

 noticed, and was led to the discovery of the fact that the pene- 

 trating power of the microscope depends upon its angle of 

 aperture " (vide Quekett's Practical Treatise on the Use of the 

 Microscope, second edition, p. 38). f Be this as it may, several test 

 diatoms are beautifully figured on PL 9 of Quekett's book, and 

 it is instructive to note that P. angulatum shown therein is the 

 Humber form with smoothly rounded outline and not the species 

 now known as P. quadratum, which, I am told, was the original 

 true P. angulatum as first found and named. Of course " diatom- 

 dotting ' was far advanced in Quekett's time. He recom- 

 mends the Navicula hippocampus as an excellent test for a l/4th- 

 inch objective-glass, stating that it should" show distinctly both 

 sets of lines or dots by oblique illumination." The younger 

 members of this club may not realise that first-class l/4th-inch 

 objectives made in 1850 have apertures slightly exceeding 0*7 N.A. 

 and will cleanly and clearly dot P. angulatum with axial critical 



* Extract from Messrs. Sollitt & Harrison's paper read before the 

 British Association at Hull, 1853 : 



" We in Hull first discovered the delicate markings on their silicious 

 coverings and pointed them out to others as the proper tests for lenses. 

 The first of the Diatomaceae on which the lines were seen was the 

 Navicula hippocampus of Ehrenberg. . . . This discovery was made 

 early in 1841, when specimens were sent to the Microscopical Society of 

 London . . . also to Mr. Smith, Mr. Ross, Messrs. Powell & Lealand. 

 M. Nachet in Paris and Professor Baily in America, the whole of 

 whom at once saw the excellency of those objects as tests for the 

 microscope. Indeed they are without doubt to the microscope what 

 the close double stars are to the telescope." E. M. Nelson. 



f First edition published 1848. 



