1900] MICROSCOPICAL JOURNAt. 15l 



resolving fine lines and details of infinitely small objects 

 when seen in the centre of the field of view, still the cur- 

 vature of the field, want of flatness, remains about the same 

 as it was at the great battle of the experts, Dr. Woodward 

 and Dr. Treadwell, at the Hayden trial in New Haven, 

 in 1879. 



Blood corpuscles seen in the centre of the field are still 

 seen more sharply defined than those in the border of the 

 field, while those seen in the border of the field are larger^ 

 less distinctly defined, and the dark ring on the border is 

 thicker, allowing less accurate measurement than corpus- 

 cles in the centre of the field. As yet opticians seem to 

 regard this as an imperfection almost insurmountable. 

 The same difiiculty occurs in the use of the microscope to 

 project images of small objects on the screen. 



But during the last few months, Spencer of Bufi"alo, has 

 constructed eyepieces for the microscope that considera- 

 bly obviate this defect and flatten the field. For several 

 years, Zeiss, Leitz and Reichart have made compensation 

 eye-pieces which greatly improve the definition of the ob- 

 jective in the centre of the field. Zeiss has at length, dur- 

 ing the past year, made other improvements in two new 

 forms of eyepieces. Watson & Son's, of London, noting 

 that the color correction and spherical aberration of no 

 two objectives are exactly alike, has made from the Jena 

 glass, by new formulae, a new eyepiece, adjustable to cor- 

 rect defects in diff'erent eyepieces. Certainly the new eye- 

 piece gives a wonderfully clear and sharp definition and 

 at the same time comes very near to giving a flat field and 

 sharp definition in the centre and in the borders of the 

 field at the same time. 



During the past year also, Reichert of Vienna, has un- 

 dertaken researches and experiments to greatly improve 

 low power objectives after a plan introduced by the writer 

 in hi* mammoth microscopic objective 20 millimeter focus 

 with aperture 0.95 N.A., made at his order in July, 1898. 



