WOODWARD, ON NOBERT's TEST-PLATE. 225 



sand mixed wdth the vegetable, from which it had been found 

 difficult to separate it. One hundred parts of pure cinder 

 contained — oxide of potassium, 15,271 ; oxide of sodium, 

 11,637; oxide of calcium, 7938; oxide of magnesia, 1915; 

 oxide of alumina, 9833 ; oxide of iron and manganese, 

 24,162; chlorine, 2445; sulphuric acid, 9232; phosphoric 

 acid, 4481 ; silicious acid, 13,115. 



Remarks on the New Nineteen-Band Test-plate of 

 NoBERT. By J. J. Woodward, Assistant-Surgeon and 

 Brevet Lieut.-Col. U.S. Army. 



In comparing the various object-glasses belonging to the 

 microscopical section of the Army Medical Museum, the test- 

 plate of Nobert has been much employed recently as the most 

 accurate means of determining defining power. The plate 

 used was one of the nineteen-band plates most recently fur- 

 nished by Nobert ; and its use for the purpose indicated has 

 led the writer to a somewhat detailed study of the plate 

 itself. 



Nobert has at various times issued test-plates with lines of 

 different degrees of fineness, the finest on the recent plates 

 being much closer than those of the earlier ones. 



An interesting account of these several test-plates is given 

 in Starting's work on the microscope.* It appears from this 

 account that the first test-plate issued by Nobert had ten 

 bands, the lines of the 1st being ruled at the rate of 443, 

 those of the 10th at the rate of 1964 lines to the millimeter. 



In 1849 he prepared plates with twelve bands, then plates 

 with fifteen, the 15th band having its lines ruled at the 

 rate of 2216 to the millimeter. In 1852 he issued plates with 

 twenty bands, the lines of the 20th band being -y-gVoth 

 of a Paris line, or -^-^j'^^th of a millimeter apart. 



This twenty-band plate has recently been described by 

 Mr. Kichard Beck, who gives an engraving which professes to 

 be a view of portions of each of the twenty bands, " as shown 

 by a -^th with number three eyepiece x 1300 linear." f 



* ' Geschichte und gegenwartiger zustand des Mikroskops,' von P. Hart- 

 ing. ' Deutsche Original Ausgabe, herausgegeben,' von Dr. E. W. Tlieile, 

 zweile auflage. ' Braunschweig,' 1866, band iii, s. 369. 



t 'A Treatise on the Construction, Proper Use, and Capabilities of 

 Smith, Beck, and Beck's Achromatic Microscopes,' by Richard Beck. Lou- 

 don, 1865. Page 19, plate 8. 



