TEST-OBJECTS FOR HIGH POWERS. 197 



ing as many separate bands of equal breadth ; in each of these 

 bands, the lines are ruled at a certain known distance ; and the 

 distances are so adjusted in the successive bands, as to form a 

 regularly diminishing series, and thus to present a succession of 

 tests of progressively increasing difficulty. The distances of the 

 lines differ on different plates ; all the bands in some series being 

 resolvable under a good objective of l-4th inch focus, whilst the 

 closest bands in others defy the resolving power of a l-12th inch 

 objective of large aperture. Thus a " test-plate" whose widest 

 lines are at a distance from each other of l-1000th of a Paris line, 

 or of 1-11, 200th of an English inch, and whose closest lines are at 

 l-5000th of a line, or l-56,000th of an inch, from each other, will 

 serve as a very fair test for the angular aperture and defining 

 power of object-glasses below l-4th inch focus ; the superiority 

 of each in these particulars, being judged of by the number of 

 bands which it will resolve into well-defined lines, and by the 

 sharpness and clearness of these lines; while the performance of 

 a l-4th inch objective may be accounted very satisfactory, if it 

 will enable them all to be clearly distinguished. But if the first 

 of the bands should have an interval of only l-4000th of a Paris 

 line, or l-45,000th of an English inch, between its lines, and the 

 last should have its lines approximated to l-10,000th of a Paris 

 line, or l-112,000th of an English inch, then only a few of the 

 easier bands will be resolved by the l-4th inch, a few more by 

 the l-8th inch, and even the l-12th inch will probably not enable 

 any band to be distinctly resolved, whose lines are closer than 

 l-7000th of a Paris line, or l-79,000th of an English inch. At 

 present, therefore, the existence of separate lines of a 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 w r ould evince the extraordinary superiority of 

 any objective, or of any system of illumination, which should 

 enable them to be distinguished. The mathematical certainty 

 with which the degree of approximation of these lines may be 

 ascertained, and the gradation of the series which they present, 

 gives to M. Robert's test-plate a very high value for the determi- 

 nation of the relative merits of different objectives, of that class, 

 at least, in which angular aperture and definition are of the first 

 importance ; whilst it also serves to test the degree in which 

 these capabilities are possessed by object-glasses of medium 

 power, in which other attributes also have to be considered. 

 The value of the minuter Diatomacece, as furnishing, in their sur- 

 face-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 endeavors 

 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 described 



