180 NOBERX'S TEST-PLATE. — DIATOM-TEST. 



plates; all the bands in some series being resolvable under a good 

 Objective of l-4th incb focus, whilst the closest bands in others 

 defy the resolving power of a l-12th inch Objective of large aper- 

 ture. The most recent of these Test-plates have nineteen bands ; 

 and their lines are ruled at the following distances, expressed in 

 parts of a Paris Line, which is to an English Inch as "088 toTOOO, 

 or as 11 to 125. 



It is stated* that these lines have been resolved by Hartnack's 

 immersion-system No. 10, and oblique light, as far as the fifteenth 

 band, in which the distance of the lines is about 1-91, 000th 

 of an inch. The existence of separate lines at a still 

 narrower interval than this, is a matter of faith rather than of 

 sight ; but there can be no reasonable doubt that the lines do 

 exist ; and the resolution of them would evince the extraordinary 

 superiority of any Objective or system of Illumination which 

 should enable them to be clearly distinguished. The mathematical 

 certainty with which the degree of approximation of these lines 

 may be ascertained, and the regular gradation of the series which 

 they present, gives to M. Nobert's Test-plate a very high value for 

 the determination of the relative merits of different Objectives, of 

 that class, at least, in which Angular Aperture and Definition are of 

 the first importance. — The value of the minuter Diatomacece, as 

 furnishing in their surface-markings admirable Test-objects for the 

 highest powers of the Microscope, was first made known by 

 Messrs. Harrison and Sollitt, of Hull, in 1841 ; and it cannot be 

 questioned that this discovery has largely contributed to the success 

 of the endeavours which have since been so effectually made, to 

 perfect this class of Objectives, and to find out new methods of 

 using them to the best advantage. The nature of these markings 

 will be discussed hereafter (§ 216) ; and it will be sufficient in this 

 place to give a table of the average distances of the transverse or 

 diagonal lineation of different species, which will serve to indicate 

 their respective degrees of difficulty as ' tests.' The greater part 

 of those which are now in use for this purpose, are comprehended 

 in the genus Pleurosigma of Prof. W. Smith; which includes those 



* See Schultze's " Archiv fur Mikroscopische Anatomie," Band i., 

 p. 305. — Prof. Schultze considers the most difficult specimens of Pleuro- 

 sigma angulatum to correspond with the 8th or 9th band of this Test- 

 plate, and the larger specimens with the 7th. 



