INVERTEBRATA, CRYPTOdAMIA, MICROSCOPY, ETC. 519 



paper. The clear aperture of front lens is always mucli less than the 

 diameter of exposed front surface which is free of the setting. 



The next column gives the diameter of field, which often varies 

 considerably with objectives of same nominal power. The next column 

 shows the flatness of field, indicated by an arbitrary standard of 1 to 6 ; 

 the latter representing an absolutely flat field, not yet found in any 

 objective examined. The next column shows the chromatic correction, 

 in regard to which Dr. Blackham says he has been unable to devise any 

 numerical method which would fairly represent this quality, and has 

 been forced to content himself with such vague notes as " slightly 

 under," " slightly over," &c. 



The next two columns give the number of the diatom on Moller'a 

 balsam-mounted probe-platte clearly and fully resolved by the lens 

 with light from lamp and mirror, and the number of lines per ■ 001 

 inch, as per Professor Morley's measurements. The next two columns, 

 the number of the diatom on same platte which could be just 

 glimpsed under same conditions, and the number of its strisc per "001 

 of an inch. The last column is for remarks. 



It is of course understood that many of the results given would 

 vary with different eye-pieces ; but all except the actual amplifying 

 power have been obtained with Tollcs' ^-inch solid eye-piece, the 

 field of which is small enough not to be affected by the size of the 

 tube of the Microscope. 



The annexed Table shows the results of the examination of forty 

 objectives of various makers. It was intended that this table should 

 be as complete as possible, but at least two important omissions were 

 discovered as the work progressed. 



1st. The diameter of the exposed face of front lens should have 

 been given. 



2nd. The number of the diatom on the Moller platte resolved with 

 direct central light, should also have been recorded. 



There are probably other points which have been overlooked ; but 

 the table is submitted as an earnest and honest endeavour to remove 

 the examination of objectives from the domain of more opinion to that 

 of carefully ascertained and accurately recorded fact. The attempt 

 has been to ascertain and record the details of the best performance 

 of each objective for itself, rather than to express an opinion as to its 

 excellence or defects as compared with some standard, ideal or actual, 

 and it is hoped that not only these records, but more especially the 

 method may prove of interest, and possibly of service, to users and 

 makers of objectives. 



Unit of Micrometry.* — At the Buffalo Meeting of the American 

 Society of Microscopists, the Committee on this subject (representing 

 a large number of the American Microscopical Societies), presented 

 the following report : — 



" This Committee, as a result of individual consideration of the 

 subject, and correspondence with Microscoj)ical Societies and students, 

 would respectfully and unanimously tender a report of progress to the 



• Rco this Journal, ii. (1870) p. 154. 



